Fix an issue in neighbors plugin which was introduced after subcategory support
Fix an issue in neighbors plugin which was introduced after subcategory support

It is not necessary that articles will have subcategories. You may
end up getting following critical error.

>
CRITICAL: ("'ArticlesGenerator' object has no attribute 'subcategories'",)
CRITICAL: 'ArticlesGenerator' object has no attribute 'subcategories'
Traceback (most recent call last):
  File "/Users/talha/Repos/VirtualEnvs/pelican-dev/bin/pelican", line 8, in <module>
    load_entry_point('pelican==3.3', 'console_scripts', 'pelican')()
  File "/Users/talha/Repos/VirtualEnvs/pelican-dev/lib/python2.7/site-packages/pelican-3.3-py2.7.egg/pelican/__init__.py", line 346, in main
    pelican.run()
  File "/Users/talha/Repos/VirtualEnvs/pelican-dev/lib/python2.7/site-packages/pelican-3.3-py2.7.egg/pelican/__init__.py", line 162, in run
    p.generate_context()
  File "/Users/talha/Repos/VirtualEnvs/pelican-dev/lib/python2.7/site-packages/pelican-3.3-py2.7.egg/pelican/generators.py", line 480, in generate_context
    signals.article_generator_finalized.send(self)
  File "/Users/talha/Repos/VirtualEnvs/pelican-dev/lib/python2.7/site-packages/blinker/base.py", line 267, in send
    for receiver in self.receivers_for(sender)]
  File "/Users/talha/Repos/pelican-plugins/neighbors/neighbors.py", line 49, in neighbors
    for subcategory, articles in generator.subcategories:
AttributeError: 'ArticlesGenerator' object has no attribute 'subcategories'

publicus/pelican-plugins,barrysteyn/pelican-plugins,MarkusH/pelican-plugins,yuanboshe/pelican-plugins,lazycoder-ru/pelican-plugins,mitchins/pelican-plugins,jprine/pelican-plugins,znegva/pelican-plugins,shireenrao/pelican-plugins,kdheepak89/pelican-plugins,prisae/pelican-plugins,xsteadfastx/pelican-plugins,olgabot/pelican-plugins,ziaa/pelican-plugins,andreas-h/pelican-plugins,benjaminabel/pelican-plugins,lele1122/pelican-plugins,andreas-h/pelican-plugins,mikitex70/pelican-plugins,rlaboiss/pelican-plugins,pxquim/pelican-plugins,doctorwidget/pelican-plugins,Xion/pelican-plugins,pelson/pelican-plugins,benjaminabel/pelican-plugins,goerz/pelican-plugins,doctorwidget/pelican-plugins,pxquim/pelican-plugins,MarkusH/pelican-plugins,clokep/pelican-plugins,jantman/pelican-plugins,howthebodyworks/pelican-plugins,clokep/pelican-plugins,barrysteyn/pelican-plugins,jantman/pelican-plugins,barrysteyn/pelican-plugins,publicus/pelican-plugins,pestrickland/pelican-plugins,Neurita/pelican-plugins,FuzzJunket/pelican-plugins,danmackinlay/pelican-plugins,karya0/pelican-plugins,rlaboiss/pelican-plugins,proteansec/pelican-plugins,prisae/pelican-plugins,jfosorio/pelican-plugins,yuanboshe/pelican-plugins,Xion/pelican-plugins,shireenrao/pelican-plugins,ingwinlu/pelican-plugins,mwcz/pelican-plugins,lele1122/pelican-plugins,goerz/pelican-plugins,UHBiocomputation/pelican-plugins,farseerfc/pelican-plugins,gjreda/pelican-plugins,ziaa/pelican-plugins,Xion/pelican-plugins,ingwinlu/pelican-plugins,howthebodyworks/pelican-plugins,Samael500/pelican-plugins,mwcz/pelican-plugins,howthebodyworks/pelican-plugins,pxquim/pelican-plugins,mortada/pelican-plugins,talha131/pelican-plugins,Samael500/pelican-plugins,jfosorio/pelican-plugins,ingwinlu/pelican-plugins,cmacmackin/pelican-plugins,shireenrao/pelican-plugins,talha131/pelican-plugins,amitsaha/pelican-plugins,publicus/pelican-plugins,danmackinlay/pelican-plugins,jcdubacq/pelican-plugins,mortada/pelican-plugins,xsteadfastx/pelican-plugins,goerz/pelican-plugins,xsteadfastx/pelican-plugins,lindzey/pelican-plugins,if1live/pelican-plugins,seandavi/pelican-plugins,wilsonfreitas/pelican-plugins,proteansec/pelican-plugins,wilsonfreitas/pelican-plugins,proteansec/pelican-plugins,gw0/pelican-plugins,danmackinlay/pelican-plugins,doctorwidget/pelican-plugins,ziaa/pelican-plugins,mikitex70/pelican-plugins,farseerfc/pelican-plugins,jakevdp/pelican-plugins,mikitex70/pelican-plugins,wilsonfreitas/pelican-plugins,jantman/pelican-plugins,jcdubacq/pelican-plugins,davidmarquis/pelican-plugins,andreas-h/pelican-plugins,if1live/pelican-plugins,davidmarquis/pelican-plugins,cctags/pelican-plugins,FuzzJunket/pelican-plugins,lele1122/pelican-plugins,ingwinlu/pelican-plugins,cctags/pelican-plugins,frickp/pelican-plugins,amitsaha/pelican-plugins,joachimneu/pelican-plugins,seandavi/pelican-plugins,goerz/pelican-plugins,if1live/pelican-plugins,Neurita/pelican-plugins,prisae/pelican-plugins,phrawzty/pelican-plugins,kdheepak89/pelican-plugins,prisae/pelican-plugins,makefu/pelican-plugins,Xion/pelican-plugins,xsteadfastx/pelican-plugins,lindzey/pelican-plugins,jantman/pelican-plugins,samueljohn/pelican-plugins,proteansec/pelican-plugins,pelson/pelican-plugins,Neurita/pelican-plugins,olgabot/pelican-plugins,jprine/pelican-plugins,talha131/pelican-plugins,jakevdp/pelican-plugins,gjreda/pelican-plugins,lazycoder-ru/pelican-plugins,seandavi/pelican-plugins,gjreda/pelican-plugins,jakevdp/pelican-plugins,pestrickland/pelican-plugins,seandavi/pelican-plugins,rlaboiss/pelican-plugins,ziaa/pelican-plugins,lindzey/pelican-plugins,clokep/pelican-plugins,Samael500/pelican-plugins,florianjacob/pelican-plugins,joachimneu/pelican-plugins,talha131/pelican-plugins,FuzzJunket/pelican-plugins,UHBiocomputation/pelican-plugins,davidmarquis/pelican-plugins,mikitex70/pelican-plugins,Samael500/pelican-plugins,samueljohn/pelican-plugins,makefu/pelican-plugins,lazycoder-ru/pelican-plugins,amitsaha/pelican-plugins,andreas-h/pelican-plugins,joachimneu/pelican-plugins,davidmarquis/pelican-plugins,cmacmackin/pelican-plugins,karya0/pelican-plugins,mitchins/pelican-plugins,jfosorio/pelican-plugins,M157q/pelican-plugins,cmacmackin/pelican-plugins,pelson/pelican-plugins,cctags/pelican-plugins,florianjacob/pelican-plugins,florianjacob/pelican-plugins,M157q/pelican-plugins,mitchins/pelican-plugins,rlaboiss/pelican-plugins,mwcz/pelican-plugins,znegva/pelican-plugins,if1live/pelican-plugins,karya0/pelican-plugins,joachimneu/pelican-plugins,Neurita/pelican-plugins,barrysteyn/pelican-plugins,frickp/pelican-plugins,MarkusH/pelican-plugins,jakevdp/pelican-plugins,florianjacob/pelican-plugins,pestrickland/pelican-plugins,karya0/pelican-plugins,yuanboshe/pelican-plugins,publicus/pelican-plugins,doctorwidget/pelican-plugins,mortada/pelican-plugins,farseerfc/pelican-plugins,pestrickland/pelican-plugins,M157q/pelican-plugins,kdheepak89/pelican-plugins,gjreda/pelican-plugins,phrawzty/pelican-plugins,lazycoder-ru/pelican-plugins,mwcz/pelican-plugins,phrawzty/pelican-plugins,UHBiocomputation/pelican-plugins,danmackinlay/pelican-plugins,talha131/pelican-plugins,benjaminabel/pelican-plugins,frickp/pelican-plugins,FuzzJunket/pelican-plugins,UHBiocomputation/pelican-plugins,benjaminabel/pelican-plugins,znegva/pelican-plugins,amitsaha/pelican-plugins,phrawzty/pelican-plugins,lindzey/pelican-plugins,farseerfc/pelican-plugins,jfosorio/pelican-plugins,mortada/pelican-plugins,samueljohn/pelican-plugins,shireenrao/pelican-plugins,wilsonfreitas/pelican-plugins,kdheepak89/pelican-plugins,olgabot/pelican-plugins,pxquim/pelican-plugins,makefu/pelican-plugins,cctags/pelican-plugins,mortada/pelican-plugins,gw0/pelican-plugins,samueljohn/pelican-plugins,pelson/pelican-plugins,M157q/pelican-plugins,cmacmackin/pelican-plugins,lele1122/pelican-plugins,yuanboshe/pelican-plugins,olgabot/pelican-plugins,frickp/pelican-plugins,mitchins/pelican-plugins,MarkusH/pelican-plugins,clokep/pelican-plugins,howthebodyworks/pelican-plugins,MarkusH/pelican-plugins,makefu/pelican-plugins,farseerfc/pelican-plugins