Add a proof-of-concept model and fit statistic in C or Cython #1095

It would be good if we could implement one model and fit statistic in C or Cython before the 0.3 release. probfit has this already and is astropy-license-compatible.

Eventually we probably want this for speed ... Sherpa has implemented most built-in models in C and even has a thread to explain users how to implement their models in C (see here).

I'm not saying we should focus on speed in astropy.modeling now before the 0.3 release ... the point would be to make sure implementing models and fit statistics in C or Cython works well with the current design.