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.
Agreed on the name change for clarity.

@msarahan I think function order it's entirely one's preference. That said, my personal preference is generally to lead with the helpers. Perhaps not everyone thinks this way, but I find glancing at them on the way down to the "main" function less frustrating instead of working on the main function and then having to go find helpers. Even though Python doesn't require, I just think in terms of "Import external stuff ... define anything else needed ... finally, use it." We've got some of both in the package.