use backbone in pdf_view.js

To resolve #2.

Everything behaves the same, but pdf_view.js is better formatted. That's not a huge change right now, but this will be super helpful once tabledetection is merged, which comes with lots of extra buttons, etc. 

We may want to refactor this even further to have one parent view for the whole interface and child views for each page of the PDF. Not sure if that would work, given how most of the things we want to implement are inter-page, like #12, applying the same lasso to multiple pages. 

I left the ZeroClipboard stuff out of Backbone, since it's weird (and small).

I can't get the debug methods to work on master, so I can't test if they work with my refactor. But those that I call don't error out, but nothing returns (or shows up, afaik).