Merge pull request #165 from matthew-brett/dicom-fix-slice-normal

BF: slice normal computing with wrong orientation

I was computing the slice normal on reversed column and row vectors; fix
and test.
DOC: document calculation of precision error

Part of the discussion of the rotation matrix thresholds on githab at:
https://github.com/nipy/nibabel/pull/156
BF: relax rotation matrix threshold

The threshold was probably too low for lower precision dicoms.

As discussed in https://github.com/nipy/nibabel/pull/156