Last.fm scrobbler

This pull request adds Last.fm scrobbler (discussed in #30). Most music players have this feature so I thought it would be useful to have it in SoundCleod as well (and the lack of it was the reason I haven't used it much).

The pull request consist of:
- FMEngine – framework for the last.fm API
- LastfmScrobbler – the scrobbler gets informed about the artist and the track, do some clean-up and checking and eventually scrobbles the track
- LastfmPrompt – last.fm credentials are entered there (currently placed in "Preferences" menu item)