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 - here and in the rest of this function, can you make of ``find_fluxvar`` as I've done now for the other functions, to avoid duplicating the error calculation code?