Niblack and Sauvola Local thresholding.

Hello,
I implemented Niblack and Sauvola, two commonly used binarization techniques. They're implemented using Integral images to speedup computation (see http://www.csse.uwa.edu.au/~shafait/papers/Shafait-efficient-binarization-SPIE08.pdf). I put them in skimage.filter.thresholding.py.

The complete list of implemented algorithms is:
- Niblack. 
- Sauvola.
- Sauvola, Wolf's modification.
- Sauvola, Phansalkar's modification.

Thank you.
Right on, @jni - the standard deviation can get a little more complicated if both addition and multiplication are present, but when it's multiplication alone these are scaled with the multiplicative factor, in this case `1 / 255.`