subpixel shift registration addition

For the sake of keeping a more readily accessibly reference to this controversial addition, here's a PR.

Discussion has thus far been at:
https://groups.google.com/forum/#!topic/scikit-image/fDbdOgHgolI

This PR should not be merged until licensing issues discussed in that email thread have been addressed.
flake8 advises me to put that space in.  If you expressly prefer it not this way, I'll change it, but I had done this to be more pep8 compliant.