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.
I think it's worth saying "(can be in either real space or Fourier space)". This both clarifies what real space means and tells the user that both types of images are supported, which is a nice feature. =)