Python tests

[tdudgeon]

Define a good approach for testing the python (RDKit) components. Currently there is a shell script that can be use to run the scripts which provides very basic testing capabilities (basically just that the scripts run, not that they work correctly), but something better is needed.
Need to work out how best to structure the scripts so that they can be properly tested.

I tried creating a python unittest (rdkit/TestScreen.py), but it doesn't work yet and there may be better approaches.