DBus communication support

Just a proof of concept, but i think supporting dbus could be a nice addition to Picard

It adds two dbus commands:
`add_directory <path>`
`debug <0|1>`

To test from shell:
```bash
$> qdbus org.musicbrainz.picard /Tagger add_directory ~/Music
$> qdbus org.musicbrainz.picard /Tagger debug 1
$> qdbus org.musicbrainz.picard /Tagger debug 0
```