Adding RectangularAnnulus aperture, and do_rectangular_photometry function

This PR logically belongs to the previous API refactoring:
 * it defines ``do_rectangular_photometry`` in ``aperture_funcs`` and moves the actual photometry computation from the ``RectangularAperture`` class, similarly to the other classes
 * defines ``RectangularAnnulus`` aperture class


@bsipocz - it doesn't really matter at the end of the day, so it's up to you :) I think the *easiest* would be to simply make sure it's rebased against master (which it is) and then address my comments above. I was thinking of merging this *before* #165 hence why rebasing on master is sufficient.