add changelog

Added in rst format, symlink is in docs
doc: link with statement in python doc

Additionally intersphinx mappings for the python
and musicbrainzngs documentations are set up for later use.

[ci skip]
doc: add currentmodule to prolog

Now we don't have to prepend discid. to every reference anymore.

[ci skip]
update MANIFEST.in

The source distribution should also contain the complete doc,
and CHANGES.rst.
[ci skip]
doc: link Ubuntu package, python2 on Arch

[ci skip]