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.  

Could you please change all asserts to use [np.assert_allclose](http://docs.scipy.org/doc/numpy-dev/reference/generated/numpy.testing.assert_allclose.html) using the `rtol` and `atol` parameters?
It leads to better messages if a test fails and by using it everywhere to more uniform tests.