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.
@stefanv (Sorry @msarahan to have you in the middle of this) I actually slightly disagree about having this function at the top. Python doesn't care about function order (unlike C), and I think public functions should be at the top of the source file, private ones coming later... Thoughts?