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.
Pretty sure the 8 minimum is to eliminate ambiguity for lines where the break is in a control statement, and immediately afterward an indented block is expected. Function declarations, complicated if criteria, etc.