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.
In this specific case, as it's followed by lines at the same level, it isn't required... I think the vast majority of the package uses 4 spaces, but I'm not sure if this is a recommended stylistic choice or a requirement.