DBus communication support

It adds two dbus commands:
add_directory <path>
debug <0|1>

To test from shell:
$> qdbus org.musicbrainz.picard /Tagger add_directory ~/Music
$> qdbus org.musicbrainz.picard /Tagger debug 1
$> qdbus org.musicbrainz.picard /Tagger debug 0