Let thread collection double as contacts db

When a thread is 'destroyed' from the UI we delete its messages and mark
the thread as inactive, (in other words, keep it around as contact info).
Additionally, we only load active threads when initializing the UI, and
reactivate threads when new messages are added to them.

Conflicts:
	js/models/messages.js
	js/models/threads.js
	js/views/conversations/show.js

js/models/threads.js

@@ -1,4 +1,4 @@
-/* vim: ts=2:sw=2:expandtab: */
+// vim: ts=2:sw=2:expandtab:
 var Whisper = Whisper || {};
 
 (function () {
@@ -9,7 +9,8 @@ var Whisper = Whisper || {};
       return {
         image: '/images/default.png',
         unreadCount: 0,
-        timestamp: new Date().getTime()
+        timestamp: new Date().getTime(),
+        active: true
       };
     },