document dictionary keys (at least for get_* functions)

[JonnyJD]

The upstream "documentation" is this:
https://github.com/metabrainz/mmd-schema/blob/master/schema/musicbrainz_mmd-2.0.rng

We do have to translate to our representation of course.

The documentation should happen in the docstring for these functions so it appears on:
http://python-musicbrainzngs.readthedocs.org/en/latest/api/#getting-data

[JonnyJD]

We already have a previous issue: #108 where I suggested to create some form of tree of elements/attributes. That would be great and easier to maintain, but more difficult to "pull off".