Python 3 support

This adds minimal support for Python 3, if that is your system Python (the one at `/usr/bin/python` or in `$PATH` during build). You can also export some variables of course to force building against Python 3. Something like: `export PYTHON_VERSION=3.3` which will cause `./configure` to use `python3.3` in `$PATH` instead of just `python`.

When creating dictionaries, keys would normally become bytes because of the direct use of `char *` . This is not desirable because then you would have to type `dict[bytes(key, 'utf-8')]` just to get a value. The keys are forced to become UTF-8 strings (the default) in Python 3.

Building against Python 2 is unaffected. Tested 2.7 and 2.6.