Added mpd's "update" command.
Added mpd's "listplaylists" command.
Added mpd's "listplaylistinfo" command.
Added mpd's "load" command.
Added mpd's "playlistadd" command.
Added mpd's "playlistclear" command.
Added mpd's "playlistdelete" command.
Added mpd's "playlistmove" command.
Added mpd's "rename" command.
Added mpd's "rm" command.
Added mpd's "save" command.
Added playlist related tests.
Added Client.readAttrsList to stay DRY.

* Replaces Client.readPlaylist
* Simplifies Client.ListPlaylists
Better function signatures and cleaner tests.

* Introduces type Playlist
* Prefixed stored playlist related functions with "Playlist".
* DRYed up some playlist related tests.
Merge pull request #6 from ushis/master

Added commands to interact with stored playlists.