Summary Diff

Currently on the summary page we have every revision being **individually** toggled. This leads to a messy layout and doesn't help when comparing revisions.

I've implemented the following logic:
- When only one revision is selected, show that **revision**
- When two revisions are selected, show a **diff**
- Only *two revisions* can be selected at the same time. It will try to automatically unselect the oldest revision and update to the last one clicked, but might not do much. You have to deselect one of the revisions and then click another one to get a new diff.
- Examples can be toggled on/off at any time on completed exercises

![Screenshot](https://dl.dropboxusercontent.com/spa/3i2zgha063bq9a2/5frn_u4g.png)

JS is a bit messier than I'd like but I went with the existing code and style. Ideally this would be extracted to an Angular.js view.