Add autocomplete in comment text areas for '@' mentions, similar to GitHub, Twitter, etc.

This commit adds autocomplete when adding an '@' mention in a comment using jquery ui autocomplete and a custom triggerAutocomplete plugin. The autocomplete is triggered by the '@' and does not interfere with other normal use of the '@', for example if entering an email address. The following screenshot shows how this looks as implemented:

![screenshot](http://i.imgur.com/sBbr0.png)