ENH: Added funtions to transform orientations.

Also added funtion to get orientations from axes codes.
DOC: Fixed typo in axcodes2ornt docstring
BF: ornt_transform was returning the inverse of the correct orientation.

Also fixed string formatting bug when an invalid orientation is passed in.
Merge pull request #146 from moloney/enh-orientations

ENH: Added funtions to transform orientations.

Work out orientation affine / coding to map from one voxel coding to another.

Calculate orientation coding corresponding to given axis codes.
BF: doctest output not correctly formatted

Doctest result for `axcodes2ornt` was correct in content, but not
identical to the doctest output.
Merge remote-tracking branch 'upstream/master'

Getting up-to-date with the main nibabel project
first working implementation