Add SoundManager2 submodule (for playing sounds client-side)
Add unread messages notifications

Fixes #3

Now the user gets a "ding" if there's on new messages if it's been at least
a minute since their last "ding".

Also the titlebar flashes betwen "DevChat" and "X New Messages!" when there's
unread messages.

This logic will have to be changed a bit.. maybe thrown into another file when
@ mentions are supported and sound preferences are supported. But for now it
gets basic and needed functionality added.