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.
The gist looks good, and I think we should use it.  I don't think I should put your gist in my test file/PR.  Since many other functions support 3D input, I think it would be nicer to include your gist in the data module, and I would then import and use it for tests here.