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.
Perhaps this is a domain-specific term.  In 1D, there's the time domain and the frequency domain.  The terms I learned as an electron microscopist for the 2D (or 3D) analogs were "Real Space" and "Fourier Space"

What terms are you most familiar with?

I don't think this particularly matters, though - I should remove this note, because the function is written to take input from either domain.