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) Update the tests in `distarray/core/tests` to "skip" rather than "pass" when there is an MPI Comm error

5) Make some additional changes by hand for 2/3 compatibility

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).  Also note that many of the tests in `distarray/core/tests` need to be run with `mpiexec`.

Note:  For some reason, I still get two test skips in `test_distarray.py`, even with `mpiexec`.