ENH: Adding vox2ras_tkr

This adds the support requested in #163.

Note that I am not 100% sure about this formulation. Based on the powerpoint linked from this page (http://surfer.nmr.mgh.harvard.edu/fswiki/CoordinateSystems) I thought that the `[-1, -1, 1]` triplet that is used to fill in the upper left corner of the matrix would need to be `[-delta[0], -delta[2], delta[1]]`, but that wasn't what the output of `mri_info --vox2ras-tkr` gave me. Can someone confirm before merging?