fix setup.py for Python 3

This should make setup.py work for Python 2 and 3.
This fixes only the file itself, but *not* the whole installation
or even makes Picard run on Python 3 already.
start compat.py

This only includes what is needed in setup.py for now.
At least encoding/decoding functions and more renamed imports are needed
later on.
more minor Python 3 fixes
Merge pull request #371 from JonnyJD/python3_setup

PICARD-588: fix setup.py for Python 3