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/9f34663433047b0367b062e0a239e85673b26a50).

Usage in a python script would then be `if "ISRC" in discid.FEATURES: ...`.