Updates for mountain lion: messages notification center

Hi Ugo-

In this branch, I've done two things:
1. enabled status setting for "Messages" - the new iChat.
2. have Notification Center display Growl messages if it is available and Growl is not installed. *Growl will always take precedence if installed. ** This will still compile on SDK <10.8, I've used dynamic class references

BTW! If you want to pull all of these branches in one shot, I've just merged them all into the "master" branch of my fork. (I won't bother you with a FOURTH pull request... sorry about that!)

I'll stop bothering you now!