Feature/sphinx and rtd documentation

This supersedes #6.

-   Add Sphinx documentation (contents from the former ``README.md`` with additional API docs.  Publish on RTD.

-   Convert ``CHANGES.md`` to RST to make them nicely embeddable in the docs.

-   Replace the old ``README.md`` with a new ``README.rst`` that points to the
    documentation on RTD.

-   Add a ``long_description`` to ``setup.py`` to provide more info on PyPI.