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


I'm not sure what is the best for this function, leave it here or move to the geometry package. I can also try to rewrite it in Cython, but don't have the time for it now.