get_features implemented as bit mask

This is option 2.1 from http://tickets.musicbrainz.org/browse/LIB-23

An example implementation using this is in python is at [python-discid](https://github.com/JonnyJD/python-discid/commit/85dedb54f3667d76daaf3c58b7fe956053fdb8b3).

Usage in a python script would then be if "ISRC" in discid.FEATURES: ....