Add simple smoothing and use it in find_cut_slices rather than a gaussian filter
Fix typo
Add fast smoothing when fwhm='fast' in image.smooth_img

* Fast smoothing has been moved from plotting.find_cuts to image.image
* Test have been added for fast smoothing
* Fix buglet where data was only smoothed in find_cuts for integer arrays
Tweak documentation and rename data -> arr for consistency
DOC: make _fast_smooth_array documentation more consistent with _smooth_array
Reduce the number of multiplications in _fast_smooth_array
Make provenance of the expected result a tad less magical
Use _smooth_array rather than smooth_img to avoid creating a temporary Nifti1Image
Add documentation for fwhm='fast' case in smooth_img
DOC: add all possible arguments for fwhm and some rephrasing + minor tweak
Merge pull request #241 from lesteve/simple-smoothing-in-find-cuts

Use simple smoothing in find_cut_slices rather than a gaussian filter