PDF figures as links from png or svg figures

I've implemented a feature that I've wanted for a while.  When a figure is displayed, the img is a link to a PDF version of the figure.  This follows the convention for webpages in my collaboration, where a figure should be displayed on the webpage as a raster, but a vector (eps or pdf) needs to be available for people to use in LaTeX.

To do this, I made four changes.  First, I added a PDFFormatter class to formatters.py, and registered it as a formatter.  Then, I changed pylabtools.py to always generate PDF output, whether `figure_format` is "png", "svg", or "retina".  I think this part still needs some work, as it is currently not configurable.  Then, I made jsonutil.py detect and encode PDF data in base64.  Finally, I added the necessary javascript to outputarea.js to wrap the anchor tag around the img tag.

I have not tested how this interacts with nbconvert, because my system does not have Haskell available.  I have to convince the sysadmins that I really need it before I can try to set up pandoc.  So I may need some assistance in that regard.

I also made these changes against the tip of the 1.x branch, rather than against the tip of master.  My apologies; I did the work before reading CONTRIBUTING.md.  I'm happy to attempt to forward-port the changes to master if need be, but I intend to continue using 1.x for the foreseeable future.

I hope that this new feature is helpful and can eventually make it into ipython.
Best regards,
Jon