Adding support for Python 3.

PYPOWER should now work on Python 2 and 3 with the same codebase. I tested it
with 2.7 and 3.3, but 2.6 should also work. I don’t think support for older
Python versions is still necessary.

I couldn’t run all tests, because that also wasn’t possible with v4.0.1, but
all tests that passed with 4.0.1 are still passing. I also performed some
power flow computations and they also worked fine.

I didn’t apply all changes 2to3 suggested. The remaining ones are in
https://gist.github.com/sscherfke/9462075 (just in case ...).