Remove IPython toolbar and replace it with our own.

refs #95 

* Moves `merge.css` link tag into the template. Before, it was being added by Javascript due to the limitations surrounding IPython Notebook extensions.
* Provides a UI element and click handler for #76 . The only thing missing for saving to work is the server-side component.
* Removes cell prompt numbers to add more horizontal space. We may start diffing these at some point, but at the moment we don't care.

This is what it looks like now:

![nbdiff-merge-toolbar](https://f.cloud.github.com/assets/305003/2150241/27af8db4-93ff-11e3-8639-fb3106c000a1.png)