update gen_gallery.py to include all examples sources in docs
update gen_gallery.py to include all examples sources in docs

- new omit_images
- update _omit(), generate_images(), generate_rst_files()

rc/sfepy,BubuLK/sfepy,vlukes/sfepy,vlukes/sfepy,BubuLK/sfepy,rc/sfepy,vlukes/sfepy,sfepy/sfepy,sfepy/sfepy,rc/sfepy,BubuLK/sfepy,sfepy/sfepy