update search fields and sort them

The search fields allowed are now the same as on the wiki in
http://wiki.musicbrainz.org/XML_Web_Service/Version_2/Search
(as of this commit)

We still don't allow searches for CD stubs, "FreeDBs" and tags.

Signed-off-by: Johannes Dewender <github@JonnyJD.net>
Merge branch 'update_search_fields'

See pull-request #98

Signed-off-by: Johannes Dewender <github@JonnyJD.net>