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.
Sorry, @msarahan, I was busy preparing to send this off when I noticed this docstring.  Can we phrase ``shift to shift image`` better?  ``Distance (in pixels) to shift `reg_image` to align it with `ref_image``` or something along those lines?  I also think we should think about the names `reg_image` vs `ref_image`--they are so similar.  Why not `src_image` and `target_img`, or `ref_image` and ...?