Added support for Pillow and darwin/linux analyzer skelleton

Modified screenshot.py to take account of the drop-in PIL replacement Pillow (it is Python 3.0 compatible and maintained) https://pypi.python.org/pypi/Pillow/2.1.0

The checks can be serial as PIL and Pillow cannot (and will not) co-exist in the environment.

It has been tested on WinXP Pro SP3 and Windows7 Pro N SP1