Fix MPD default constructor

As reported in issue #15
Converted Database results from Collection to List

This change doesn't cost anything and it allows us to access the results
via index, sorting, sublist, etc..
Converted MPD status entries from List to Map
Fixed NPE in MPDPlayer.getBitrate()

It now returns 0 if no song is playing
Fixed MPDSong.getName() and getTitle() inconsistency
Fixed MPDSong equals() and hashcode()

The methods getPosition() and getId() could be different and equals()
still reported true. If a song exists twice in a playlist, sometimes the
wrong one gets deleted.