Add audioresample and queue to pipeline, fixes #159

- Audioresample should perform as a noop in cases where no conversion is
  needed. In cases where the sink requires a fixed sample rate this will
  prevent output from breaking.

- The queue is needed to ensure that our outputs play nicely and is simply a
  continuation of the queue that was in our old Output abstraction.

mopidy/gstreamer.py

@@ -52,6 +52,8 @@ class GStreamer(ThreadingActor):
         description = ' ! '.join([
             'uridecodebin name=uri',
             'audioconvert name=convert',
+            'audioresample name=resample',
+            'queue name=queue',
             'volume name=volume'])
 
         logger.debug(u'Setting up base GStreamer pipeline: %s', description)