After a message is saved asynchronsly, fire an event and pass the
message attributes to frontend listeners via the chrome-runtime API.
This behavior is similar to the 'storage' event fired by localStorage.
Getting up and running with IndexedDB was pretty easy, thanks to
backbone. The tricky part was making reads and writes asynchronous.
In that process I did some refactoring on Whisper.Threads, which
has been renamed Conversations for consistency with the view names.
This change also adds the unlimitedStorage permission.
