ENH: Allow use of nibabel

This gives equivalent results to using `mri_info` with a dependency on `nibabel` instead of Freesurfer. Should be a little quicker, too, which is nice.

Before merging, someone should double-check that the `vox2ras-tkr` transform I calculated is correct according to:
http://surfer.nmr.mgh.harvard.edu/fswiki/CoordinateSystems?action=AttachFile&do=get&target=fscoordinates.pdf