Firgure out how to install the C headers properly #116

According to the documentation (https://docs.python.org/2/install/), C headers that come along with an extension should be installed according to the scheme chosen on command-line (user, home, prefix or custom scheme).

For example, here are three possible location.

userbase/include/pythonX.Y/distname
home/include/python/distname
prefix/include/pythonX.Y/distname

Currently, the setup.py script file force the installation in /usr/local/include/ (resulting in something like /usr/local/include/pysfml/system.h).

The question is, how do we let distutils take care of installing the C headers ? Right now, C headers are treated like "additional data files" and that's how they get installed.