last.fm scrobbling support

[chrismou]

Support for scrobbling. Ideally this will use their v2 API (oauth) but if required we can fall back to v1 (username/password).

Oauth2 is preferred, but bear in mind that going down this route will mean the authentication logic can't be shared with libre.fm (#5). However, I believe the credentials methods have been deprecated (though they're not actively enforcing this, and most likely they won't be switched off any time soon due to the sheer number of apps that will break).

• Essential:
  -- Make "Now Playing" call to last.fm when song starts
  -- Scrobble song at 50% (or at the very least, when the song changes and we know at least 50% has been listened to. Ideally the former).
• Nice to have:
  -- Option to a custom percentage before scrobbling. Minimum 50% (as per last.fm guidelines). Default to 50%.

Depends on #3 for somewhere to put the options