replace non-free xmlparser with libxml2

This PR rebases the patches from Christophe Fergeau on the current master branch and fixes some outstanding issues. It also bumps the SOVERSION to be on the safe side.

I've test built all reverse of libmusicbrainz in Debian with these changes and didn't see any build failures. The output of the included test programs looks sane to me.

Please let me know if there are any outstanding issues. It'd be great to get this fixed before the Debian jessie freeze (2014-11-05).