Add roles support for ACL manager UI

This PR completes the ACL manager UI by adding roles support.

![acl_editor](https://cloud.githubusercontent.com/assets/4034907/5253042/ac4a4e12-79a3-11e4-9e60-f4cc5c16db67.png)

cc @dunglas 

@meyerbaptiste Can you please add this new translation string to all transaltion files (can be `td_role`)?