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.
@msarahan the code is so much clearer than it started out! Again with the caveat that I don't really understand this step, do you think the [tensor dot product](http://docs.scipy.org/doc/numpy/reference/generated/numpy.tensordot.html) is the way to generalize this to nD? (Not that we have to do it in this PR, but thinking out loud while reviewing.)