contributing C/Cython code to the scikit

[mael]

Thanks Emmanuelle for your code refactoring and the remarks,

> * if there is no good reason to keep lena256.tif (if you can use
>   scipy.lena() in all your examples and tests), then why not remove the
>   file.
sure

> I did not have the time to read thoroughly the code of your median
> filter, I think you wanted to know if this was efficient Cython code or
> not. I'll do it soon, maybe after you write a test function for this
> filter? By the way, what if the advantage of this function compared to
> scipy.ndimage.median_filter?
No there is no reason to use our own rather than scipy's one (except
that my implementation might be less obfuscated
because it is less complicated) it was more an example of what we
could do to bind C and python