Unified support for Python 2 and 3

Make the codebase compatible with Py2.7 and Py3.3!

1) Convert existing tabs to spaces (Python 3 doesn't allow tabs and spaces in one file)

2) Do an initial conversion of the codebase with `python-modernize` that adds a dependency on `six`

3) Clean up `python-modernize`'s output a bit (remove repeated `__future__` imports)

4) Make some additional changes by hand
  * change references to `ConfigParser` (in `setup.py`) to point to `six.moves.configparser`
  * 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).