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.
The only thing that makes me hesitate is that they also make the gallery example much more interesting to look at.  Without them, it's just the numbers.  Cross-correlograms are pretty.