Python 3 support

Following discussion on PR #32, this adds Python 3 support without any external dependencies. Python 2.6 and above and Python 3.3 and above should work; older versions will not.

The tests are passing on Python 2 and 3; I haven't tried the examples.
(general comment) What happens in Python 2 when the L is dropped? Do we lose some bytes that we had before?
How can we make sure, with tests, that this is having no effect under Py2 and does the right thing under Py3?