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). I thought I'd share them as they could be generally useful to PyStemmer.

Sample run (run in a virtualenv with `Cython` and `nose` installed):

```
(venv)marca@marca-mac:~/dev/git-repos/pystemmer$ nosetests -v
test_stemWord (test_pystemmer.PyStemmerEnglishTests) ... ok
test_stemWord_many_times (test_pystemmer.PyStemmerEnglishTests) ... ok
test_stemWords (test_pystemmer.PyStemmerEnglishTests) ... ok
test_stemWords_unicode_simple (test_pystemmer.PyStemmerEnglishTests) ... ok
test_stemWord (test_pystemmer.PyStemmerFrenchTests) ... ok
test_has_algorithms (test_pystemmer.PyStemmerGenericTests) ... ok
test_has_version (test_pystemmer.PyStemmerGenericTests) ... ok
test_import (test_pystemmer.PyStemmerGenericTests) ... ok
test_stemWord (test_pystemmer.PyStemmerGermanTests) ... ok
test_stemWord (test_pystemmer.PyStemmerHungarianTests) ... ok
test_stemWord (test_pystemmer.PyStemmerRussianTests) ... ok

----------------------------------------------------------------------
Ran 11 tests in 0.094s

OK
```