MRG: add resampling and smoothing functions

This pull request adds some simple processing functions when scipy
is available. The functions are:

* resample_from_to - resample first image into space of second
* resample_to_output - resample image into output (world) space
* smooth_image - smooth image over voxel axes

I also put in the utility functions converting between FWHM and sigma.
Should any docs be updated to mention this? Not sure what `nibabel` doc building / standards are like.