get_features implemented as array of strings

This is option 2.2 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/60f4b6833c5b597ad3ca2baea538690e7d0bf648).

Usage in a python script would then be `if "ISRC" in discid.FEATURES: ...`.