Include Cython generated .c files in the repo

This separates the generation of the .c files from the actual compilation.
So Cython is no longer required to build Fatiando and we can avoid all the error due to incompatible Cython versions (specially with memoryviews).

This is recommended in the Cython docs: http://docs.cython.org/src/reference/compilation.html#distributing-cython-modules