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.
Thanks, Juan.  You all have been very helpful with your suggestions, and I'm grateful for your patience.

I think that the tensor dot product is the way to generalize it, but I don't know how to work with tensors.  I'm reviewing MIT's linear algebra open courseware to pick up linear algebra, and hoping it will be in there.  College was only calculus for me.  It is frustrating to have a clear mental picture of what I want to do, but no expression of that idea.