The MPD server has a connection_timeout that defaults to 60. If no data
is transferred in this timespan, the connection is closed. If the
connection is closed while the event listener is listening for events,
it will block forever. By timing out the event listening and
re-connecting regularly, we prevent this issue. An option "timeout" has
been added for users that have a lower server connection_timeout than
default. Fixes#277
Because `mpd_song_get_tag` from libmpdclient can return NULL, verify the
value of tag is valid. Otherwise, set a default string of "N/A". Also
adds configuration to specify what this default string should be.
