Make facet label edit-in-place.

Closes #535 

![facet-admin](https://f.cloud.github.com/assets/96776/2455541/2c4a87b2-af01-11e3-9aee-d42d1ef443c7.png)

![edit-facet-admin](https://f.cloud.github.com/assets/96776/2455542/2f4bc5ca-af01-11e3-976b-53d180dd0df8.png)