[WIP] NL-means denoising

Following a message on the mailing-list, here is a first attempt at contributing my non-local means denoising code. It still lacks an example for the gallery. I would also welcome suggestions to improve the speed, since this algorithm is very slow...
that's right. Right now, with your "result" initialization it is as if you had doubled the weight of the pixel of interest in its own averaging. It does not mean it was worst in practice ;)