Make the codebase compatible with Py2.7 and Py3.3!

First, do an initial conversion with `python-modernize` that adds a dependency on `six`.  Then, make some changes by hand:

1) change references to `ConfigParser` (in `setup.py`) to point to `six.moves.configparser`
2) change `foo.get_hex()` to `foo.hex` in `client.py`

Tests now pass in both Python 3.3 and Python 2.7.  Note that you must recompile the extension modules for each Python version, and you must run the correct version of `ipcluster` (namely, `ipcluster3` for Python 3 in my installation).