replaced command functions with generators; removed response objects
replaced command functions with generators; removed response objects

(ErrorResponse functionality is now in BPDError.) This was necessary
because libmpdclient was timing out waiting for the results of
long-running commands like listallinfo. By sending data as it is
generated, we get data to the client more quickly.

google-code-export/beets,google-code-export/beets,google-code-export/beets