Move message and conversation storage to IndexedDB

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.

js/views/message_list_view.js

@@ -13,5 +13,12 @@ var Whisper = Whisper || {};
     scrollToBottom: function() {
         this.$el.scrollTop(this.el.scrollHeight);
     },
+    addAll: function() {
+      this.$el.html('');
+      this.collection.each(function(model) {
+        var view = new this.itemView({model: model});
+        this.$el.prepend(view.render().el);
+      });
+    },
   });
 })();