Implement Metadata Tag inject to fix #178

This uses the previously commented gst.taginject code to output information about the audio stream using standard metadata. There is a new function RoundStream.set_metadata() to set the current metadata.

Plus, finish renaming Composition to AudioTrack.

This might be done? Looks good tests pass and VLC shows the correct metadata in the titlebar when stream-test.py is run. Need anything else?