Add Raspberry Pi support

[zsquareplusc]

When this library gets some new life after all... there should be a back-end specifically supporting the GPIOs on the Raspberry PI (and maybe additional embedded boards).

Does not need to be very flexible, and more or less provide only minimal interface mimicking the parallel port functionality found on the PC.
For full featured GPIO access on RPI, there are already enough libraries (which we do not need to duplicate) such as:

• https://pypi.python.org/pypi/RPIO
• https://pypi.python.org/pypi/RPi.GPIO