Christmas can come twice a year

Modifying the validation for the scrobble API calls to match the one for
now_playing. Now ListenBrainz has a separate validation which doesn't
take into account the album name, which can be omitted from the request.

Additionally we fixed an issue where submissions to last.fm failed for
some users because our request signature was bein truncated.

Thanks again to the users that brought these issues to my attention:
@MaxEtMoritz and @ChandlerSwift

Full Changelog: v0.5.4...v0.5.5