Compatibility with Python 3

I needed to use python-scss in Python 3 project, so I decided to make it compatible with both Python2 and Python3.

I created `compat.py` to keep in one place as much of the compatibility workarounds as possible.

Unittest results on my py3k branch are the same (with both Python 2.7 and 3.3) as on the original master branch.

Comments and improvements are welcome.