Modernize supported Python versions.

- Remove fossil test dependency on deprecated ``distribute``.

- Add explicit support / testing for Python 3.3 / 3.4.

- Drop explicit support for Python 2.5 (out of maintenance, and no longer
  supported by ``tox`` or ``Travis-CI``).
Merge pull request #173 from tseaver/add_py33_py34-drop_py25

Modernize supported Python versions.