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.
FYI, [per the NumPy docs](http://docs.scipy.org/doc/numpy/reference/generated/numpy.testing.assert_array_almost_equal.html), `assert_array_almost_equal` should pretty much never be used in favor of better array comparison functions in tests. `assert_allclose` is one of the recommended ones, and my personal go-to.