Add getPrevContent function and mark it's situational presence.

lib/models/event.js

@@ -91,6 +91,15 @@ module.exports.MatrixEvent.prototype = {
         return this.event.content || {};
     },
 
+    /**
+     * Get the previous event content JSON. This will only return something for
+     * state events which exist in the timeline.
+     * @return {Object} The previous event content JSON, or an empty object.
+     */
+    getPrevContent: function() {
+        return this.event.prev_content || {};
+    },
+
     /**
      * Get the age of this event. This represents the age of the event when the
      * event arrived at the device, and not the age of the event when this