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 that's one decimal point, not the nearest integer.  Still subpixel.  In my experience, the limits of precision are ~.02 to .03 pixels.  No amount of preprocessing I've done has improved things for me.  Thus, I have the numerical equality test to 1 decimal place.  Maybe a better metric would be (error < some magnitude)?