Add MANIFEST.in.

Without a ``MANIFEST.in`` file, ``python setup.py sdist`` does not include ``CHANGES.txt`` or ``README.rst`` in the packaged sdist, which breaks installation because ``setup.py`` references these files. This pull request fixes that.

I also chose to include the documentation in the packaged sdist, as I generally consider that a good practice, but this could be removed if you don't want it.

I presume you must not be using ``python setup.py sdist`` to generate the sdists on PyPI, or they would be broken too?