Rename Bad_response -> Parse_failure

Signed-off-by: John Else <john.else@citrix.com>
Fix Mpd_parser.parse_ok

Previous implementation didn't handle the case where the
response is just "OK\n"

Signed-off-by: John Else <john.else@citrix.com>
Implement ping

Signed-off-by: John Else <john.else@citrix.com>