Python 3 support

You might be interested in some changes I've made to see how Parsley can work with Python 3. The work was straightforward (a couple of hours) and all tests pass on 2.7 and 3.2 using a single code base which runs unchanged on >= 2.6 and 3.x. (The changed code will not work on Python <= 2.5 because of the incompatible exception syntax - though this could be hacked around, it makes the code ugly, so I didn't do it. I couldn't see from Parsley's docs or PyPI metadata what minimum Python version it works with.)

Even if you have no interest in merging these changes (as you don't use Python 3), I would be grateful for any comments you have.