Support for Python3

There were no major changes that needed to be made to make the library work with Python3 except for renaming `uritemplates.py` to `__init__.py` because of the new absolute importing rules. Other than that, I only updated the test runner and replaced the instances of list.sort() with sorted(list).

Tests pass on 2.5, 2.6, 2.7, 3.3 as well as pypy.