tox.ini: Remove py25

Python 2.5 is old. Tox and virtualenv don't support it anymore, IIRC
tox.ini: Measure test coverage
Increase test coverage to 100%
tox.ini: Test pypy, py33, py4
Work with Python 3 (and PyPy)
Merge pull request #13 from msabramo/py3

Work with Python 3 (and PyPy)