Merge pull request #15 from agriffis/master

Port to Python 3.

The code now works on Pythons 2.5--3.3, thanks to Aron Griffis.
Bumped to v2.3.0.
Vendorized six.

Also added a credit to Aron Griffis for the Py3K port.
Fixed tests and impl. for URLPath.join_segments().
Refactored Py3K compatibility code to use six.
Bumped to v2.3.1.
Remove duplicate code.
Bumped to v2.3.2.