More Cython bindings, cleaner unsharp_mask

Still missing the Check test of unsharp_mask, etc. but when I test from Python, all seems to work as expected. I'd be willing to think about 16bit image treatment in a separate branch for two of us, while the existing code can be submitted to the OpenPTV community for the review (it goes so slow that we'll probably finish all our duties long before they accept anything :))

Meanwhile, we could generalize our handle_borders - which is good only for 3 x 3 filter now and think about the solution of the filter_n which is still strange for small images. As we do not deal with small images, maybe it's just a drawback of this fast algorithm. Again, Python use of Lena image shows it simply works. 

Shall we submit as is? If you review and it works on your computer (Windows?) I'd be glad if you can submit the pull request.

Thanks,
Alex