Add support to edit collection backgrounds

Adds support to edit collections. This initial check-in adds support to change the background image, a subsequent check-in will add more functionality. See discussion here: https://github.com/alterm4nn/ChronoZoom/issues/1235

Additionally, this change:
- Adds support for line breaks in the description field (user ask)
- Navigates directly to the timeline for collection pages (user ask)

![modifycollectionbackground](https://f.cloud.github.com/assets/3478847/2027562/9619ae4e-88b8-11e3-87c0-1e763c7fc34b.png)