support for unpacking megolm keys

This is incredibly hacky at the moment, pending the arrival of ephemeral
events, but it kinda works.

lib/models/event.js

@@ -232,6 +232,14 @@ module.exports.MatrixEvent.prototype = {
         return Boolean(this._clearEvent.type);
     },
 
+    getSenderKey: function() {
+        if (!this.isEncrypted()) {
+            return null;
+        }
+        var c = this.getWireContent();
+        return c.sender_key;
+    },
+
     getUnsigned: function() {
         return this.event.unsigned || {};
     },