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.
This dependency may be a little heavy for our current use case, but we can
roll with it for now and find something slimmer if it turns out yagni.
Closes#77Closes#40
