Add a Python style guide

[jhlegarreta]

Add a Python style guide.

The System Overview Wrapping section contains some example code of what an ITK Python code looks like, but we are lacking some guidelines on how to write proper ITK software using Python.

Besides, ITK supports more than one syntax or interface, and the ITKExamples seem not to have converged to one yet.

We should may be provide an overview and encourage using one.

Also, given the importance of the Python wrappers, the Python section may need further refactoring.