Add unittest tests that supplement current doctests and go a little bit further in testing more cases.

While working on figuring out the Python 3 compatibility issues described in #2, I wrote some tests to verify basic behavior. These go a little further than the current doctests as they do things like test multiple languages (including some with Unicode characters) and test with a large number of English words (in order to exercise the [cache purging that is failing for me with Python 3](#2)). I thought I'd share them as they could be generally useful to PyStemmer.