Python 3 support, fix #2.

Hey how are you! Let's get this forward compatible with python 3. #2 

- ``__future__`` imports for ``.py`` files.
- Replaces some instances of ``filter`` with list comprehensions.
- Created a ``py2map`` inside of ``_compat`` to import into code, which preserves python 2.x's ``map`` behavior in python 3.
- Update README.rst for python 3 support
- Update ``setup.py`` classifier data
- Update Travis for python 3.3 support (they don't have 3.4 support yet, tox passes 3.4 for me though)
- Import ``reduce`` from ``functools``.
- Add ``tox.ini`` file.