Include tests in release source tarball

It would be great to include the test suite in the pypi release tarball.
That way people who download and build the release can run the tests to
ensure the library is working.

Linux distributions, such as Debian, base their packages of Python
modules on the pypi release. This means the test suite can be run when
building a Debian package, it should catch mistakes in the packaging or
errors in dependencies.

I've written some more on this topic on the Debian Python mailing list.
https://lists.debian.org/debian-python/2016/04/msg00074.html

MAINFEST.in

@@ -1,2 +1,3 @@
 include LICENSE
-include README.rst
+include README.rst
+recursive-include tests *.py *.md