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.
Actually, can we rename this? e.g. `_upsampled_discrete_fourier` or `_upsampled_dft`? `_dftups` is very mysterious!