Added a wrapper around ndimage's Gaussian filter

This version of the Gaussian filter

 * uses 'nearest' as the default boundary mode. This can be discussed,
   but I had the impression that for images this is the most relevant
   mode ('extending' boundaries)

 * has a `multichannel` keyword, so that each color channel can be
   filtered separately. For now no attempt is made at guessing whether
   the image has color channels or not.
DOC: corrected description of multichannel parameter
[BUG] iterable sigma and multichannel

+ docstring improvement
Gaussian filter function: changed the default value of multichannel

Now we try to guess automatically whether the image is grayscale or RGB
Merge pull request #712 from emmanuelle/gaussian_filter

Add a wrapper around `scipy.ndimage.gaussian_filter` with useful default behaviors.