ENH: Added Cython implementation of innerloop

Hi Jesus,

Interesting idea - was curious about the performance of Cython, which is a mature Creole for Python, on your problem.  "Cythoning" code is very simple (but not as easy as Numba) but provides a some performance continuity for people who find simple loops too slow in Python.  Performance is pretty good, with a relative runtime of

```
Python-Cython               1.61
-------------  =            ----    =     89%
Python-Numba                1.81
```
Feel free to add this if you think it is useful.  I release it under same as in original project MIT.  

I have some experience at Cythonizing code, but it only took around 15 minutes to achieve this performance (which is still about 14 minutes longer than it would take to use Numba).

From the commit message:
Added a Cython version of the innerloop as an alternative to Numba.  Also added
a setup file to compile the Cython version and a wrapper that uses the Cython
version in-place of the Numba innerloop.