Changing aperture photometry output to return astropy.table

This is the first draft for returning the photometry results in an ``astropy.table``.
I think the main question is what we want to return. Only flux, fluxerr, or something more, e.g. positions, or even the apertures (as metadata), etc.  

I leave this one now, but try to keep it in mind when adding the coordinates to the output table (probably in #122)