Groups v2

This PR add the following groups features:

1. Delete Group with confirmation
2. Cascading delete when deleting a group
3. Group link
4. Reset link feature
5. Basic group list on /groups. Add menu item.

![groups](https://cloud.githubusercontent.com/assets/357127/2922613/06941c10-d704-11e3-8d2d-0e1e8dde492e.png)