add-bottom-collapsible

- Adding CSS and Jquery to add the collapsible div at the bottom of each diff.
- The newly added div reads "Collapse diff" and clicking on it will collapse the diff
- Once clicked the added div disappears
- Expanding the diff again re shows the div
- Screenshot:
![prettypullrequest](https://f.cloud.github.com/assets/5394747/1747821/8a38896e-6479-11e3-8fbb-4c916c200f66.png)

@byates-va