Add ISRC / MCN API

I won't do this in beta state, because this is only in libdiscid 0.3.0 (which is quite new) and there are still logical issues with that API.

There is no way yet to tell if the platform doesn't support ISRCs (with libdiscid) or there are no ISRCs on the disc.

See: http://tickets.musicbrainz.org/browse/LIB-23