setup.py: Don't install the 'tests' package

This package is useful for testing, but not namespaced appropriately
for a system-wide install ;).  For docs on the exclude syntax, using
this exact situation as an example, see [1].

[1]: https://pythonhosted.org/setuptools/setuptools.html#using-find-packages
MANIFEST.in: Distribute the tests package

We don't want to *install* these, but we do want users to have them in
the tarball, in case they want to run the tests locally to verify
their installation.
Merge pull request #18 from wking/tests

Cleanup tests/ packaging