Clean C/Cython API

[intjelic]

I'm not sure if there's an official location for C/Cython headers or if they even thought about sharing Cython definitions to reuse later in other project such bindings depending on other Cython bindings. Anyway I need to share some files across projects (pyThor pySFGUI, pysfeMovie) and their location should be standard. They also need to follow fixed conventions (do we add "py" before project name, so we would import header with cimport py<name>.<module name>, etc.). I also had a clash name when porting Thor to our bindings with the STL. One module is named "Math" and Cython fails to detect the right header (mixed stl location and project include location).