Mentions implementation

This PR implements mentions ala github, making it easier for users to mention buddies or just other forum users.  Thanks goes to @ichord for his awesome mentions auto-complete script: https://github.com/ichord/At.js

- It queries the DB to get a list of possible mentionable users, there is an admin setting to determinate how many posts does an user needs to be mentionable, this query is cached to prevent server overheat
- User setting to determinate how many users will appear on the list of possible mentionable users
- Auto-complete script injects a per-formatted string: @(username, ID) back to the textarea that gets parsed by BreezeMention to get the data and send the notifications and in BreezeParser to convert it to a user link.
- binds an ajax call to all the textareas in the profile based on "focus" and fires a new instance of atwho only on success, gotta implement a notification when an error is returned instead.
- Removes a query to check the validity of the entry, I might need to add the check again to avoid possible abuse of the system.