Task 3125: mentions notifications

@tanius @albertocottica here you can find a first draft of the implementation for mentions notifications.

The implementation is actually based on:
1) Message
2) Message notify

Entity structure:
- Subject
- Body
- Node/Comment ref.
- Users ref <list>. (mentioned users)

Each notification is saved as a message entity, this means that is virtually possible to build views on top of it (for example a page with nodes where i have been mentioned).

Users can disable mentions notifications unchecking the option within the notification settings page.
![screen shot 2014-08-13 at 18 58 31]
(https://cloud.githubusercontent.com/assets/8747/3909193/24e48c10-230b-11e4-887d-d803dbb4d4fc.png)