Folders UI

Initial PR to get a sanity check on the work done so far.

The following items have not been tackled yet:

- Changing the visibility of a folder
- Deleting folder(s) from the system
- Adding existing items to a folder
- Change the URL when switching between the content and folders tabs
- A large part of the tests
It looks like the 2px margin-left is necessary. It's easiest to see when looking at a small thumbnail:

Without:

![screen shot 2014-09-02 at 13 51 57](https://cloud.githubusercontent.com/assets/109850/4118349/0952f0dc-32a0-11e4-806d-2da2c0898f48.png)

With:

![screen shot 2014-09-02 at 13 51 53](https://cloud.githubusercontent.com/assets/109850/4118348/06b697ca-32a0-11e4-8d31-003eafe6bdbb.png)