Python3

I have made some changes to make this package work with Python3.  However, it is not backwards compatible with Python2 so it will need to live in a separate branch until I manage to make it work for both.

I used the 2to3 utility for the bulk of the changes. But I also had to hack away at the pairtree_path.py module. The elegant regex solution doesn't work well in Python3 because you can't mix bytes with unicode. I think my solution could be improved upon, but for now it works and passes all tests.