Quick Reference PDF link addition

Hello, 

I made the changes suggested by tookitook (not me) [here](http://www.reddit.com/r/IPython/comments/1dh81f/ipthon_quick_reference_pdf/c9qws56) on the IPython subreddit.  The [PDF](http://dl.dropboxusercontent.com/u/54552252/ipython-quickref.pdf) that is hosted on dropbox (not my pdf) was added as a link to the documentation page.  If you would prefer I can host the PDF in my dropbox but I thought that the original poster may make useful future revisions.