Added 'audio' method to ChannelMap

Added `audio` property to channelmap to test if it contains any audio channels.

pycmx/channel_map.py

@@ -27,6 +27,11 @@ class ChannelMap:
         'True if video is included'
         return self.v
 
+    @property
+    def audio(self):
+        'True if an audio channel is included'
+        return len(self._audio_channel_set) > 0
+
     @property
     def channels(self):
         'A generator for each audio channel'