Beginning of setup.py, following https://github.com/vene/pydai

Following @vene, compiling c library and then linking the cython extension to it. 

Seems to be working.

TODO, 

- [ ] decide on the namespace for the library, pystruct is expecting ad3, which conflicts with the directory structure of AD3, since the cpp source files are in that directory.
- [ ] Cythonize factor_graph.pyx so that cython is not a dependency for installing this library