Notifications

First of all: it has few issues and there is still some work to do, so **not** merge it!

This PR introduces two things:

1. a "notifications" system (i.e. a way to "notify" users of something that "happened" for example if another user liked/unliked one of their messages)
2. a mentioning thing (@ someone with a github-like droppy with usernames that will generate a notification that will be added to 1)

Issues:

1. Notifications are not filtered by permission access (that's important)
2. Opera plain textarea in quick reply has an offset problem (since it's rather localised we can put a workaround quite easily),
3. WYSIWYG doesn't work, two are the issues:
 1. the dropdown is misplaced because the position is calculated based on the body tag inside the iframe (I think)
 2. when an item is selected an error is generated and the ends without doing what it should.

things to do:

1. button to reach the notifications page
2. show notifications more "in front of your face" (and do something more with those notifications)
3. "revalidate" the mentions before post the data
4. convert the @ to some kind of bbcode
5. the above mentioned bbcode

Apart from all that, I'm out of ideas on how to fix the js and so this is it.
I'm not sure it's feasible to have it working for 1.0, if not we can make it a plugin and add it to 1.1.