Added a new RPC to get file offsets of messages by hash or of last 2 checkpoints, also improved checking of valid channel names and fixed a pull-stream bug and exposed async-store to the window

www/common/outer/async-store.js

@@ -24,7 +24,7 @@ define([
 
     var storeHash;
 
-    var store = {};
+    var store = window.CryptPad_AsyncStore = {};
 
 
     var onSync = function (cb) {