add cython_gensetup_py: utility script for generating setup.py

I kept copy-pasting from the [userguide](http://docs.cython.org/src/userguide/source_files_and_compilation.html), so I thought I should automate that.

I'm not sure if this is following current conventions, as the [C++ section of the userguide](http://docs.cython.org/src/userguide/wrapping_CPlusPlus.html) suggests a different way of writing the `setup.py`. Maybe the docs should be updated to reflect this?