Add unit test python files to release package.

[tovrstra]

This makes it possible to run the tests from the source release. It would even be better to make the tests a subpackage of the unidiff main package. That would make it even easier to run the tests after installing then with pip: python -m unittest unidiff.test

[kapilt]

this is pretty uncommon to do in most libraries or applications, i'm not sure whats the value proposition.