Update the Python version classifiers, and remove use_2to3

staticjinja supports Python 2.6, 2.7 and 3.3.

This change also removes support for Python 2.5, which isn't
supported by tox (so ensuring staticjinja works with 2.5 is
tricky).
Python 3.4 also works, so we should test it
Add note in the documentation about supported Python versions
Merge pull request #31 from dominicrodger/py3

Document and test support for Python 3.4, deprecate Python 2.5