setup.py fixes  - update long description to refer to readthedocs  - fix download_url, which was missing a /
setup.py fixes
 - update long description to refer to readthedocs
 - fix download_url, which was missing a /

juhasch/IPython-notebook-extensions,ipython-contrib/IPython-notebook-extensions,juhasch/IPython-notebook-extensions,ipython-contrib/IPython-notebook-extensions,juhasch/IPython-notebook-extensions,jcb91/IPython-notebook-extensions,ipython-contrib/IPython-notebook-extensions,jcb91/IPython-notebook-extensions,ipython-contrib/IPython-notebook-extensions,jcb91/IPython-notebook-extensions