Some important spec changes

[fluffy-critter]

To improve compatibility and usability of the feeds, I've made a few important changes to the spec, namely that all keywords are to be formatted as all lowercase using underscores as the word separator. This makes it so that people can be guaranteed to do a keyword lookup with attribute names.

The change is documented at 61d0561.

Also, for those who have been using the Sockpuppet feed as a reference, there's some attribute names in it which were never correct to begin with.

I've also removed the musicbrainz-related things from the spec for now, because it was a bit half-baked and wouldn't have actually been useful as designed. It's something I'll definitely want to add back in later but it needs a bit more fermentation first.