Fixed whisper messages

* Whisper protocol wasn't properly suppling envelope slices
* Message history wasn't properly propagated
* Added 'Messages' method, filtering any current envelope with the
  supplied filter.

cmd/mist/assets/qml/browser.qml

@@ -357,7 +357,9 @@ Rectangle {
 			case "shh_getMessages":
 				require(1);
 
-				shh.trigger(data.args[0]);
+				var m = shh.messages(data.args[0]);
+				var messages = JSON.parse(JSON.parse(JSON.stringify(m)));
+				postData(data._id, messages);
 
 				break;
                     }