Use features API from libdiscid

As of [LIB-23](http://tickets.musicbrainz.org/browse/LIB-23)/libdiscid 0.4.0 there is an API to check if a certain feature is actually implemented on a platform.

Currently: read, mcn, isrc

For 1.0.0 we only have to check for read, but we expose the whole list anyways.

We should already give NotImplementedError when reading and the feature is not set.