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.
Would it make sense to also compare the output of these algorithms to the output of an equivalent operation performed in a standard packge (fsl?)?