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.
They should know based on how they set the upsample_factor parameter.  What would be a more clear way?  Embedding the upsampled image doesn't make sense to me.  Would it be better to have another two input args for storing outputs, if present (as handles?)

These images probably aren't generally useful, but they are informative when trying to figure out why things might be going wrong.  For example, the earlier quarter-pixel problem I had was because the subpixel part was sampling the wrong area, and examining the upsampled output made this clear.