Data for WCSAxes

We don't have to merge this yet, but this is the first step towards moving datasets towards a centralized location and I wanted to have a discussion about the layout we should adopt. I was thinking that we could simply have these directories at the root of data.astropy.org:

* ``images`` - celestial images in any format
* ``catalogs`` - catalogs in any format
* ``spectral_cubes`` - spectral cubes in any format

and we can add more categories as needed. I originally had split the root level into packages but then of course some datasets could be used by several packages so I think the above option makes sense.

@eteq @cdeil - what do you think?