Rename astropy models to modeling and builtin_models to models

I renamed `astropy.models` to `astropy.modeling` and `astropy.models.builtin_models` to `astropy.modeling.models` as discussed in #997 and in yesterday's telcon.

@nden, @eteq Please have a look and let me know if I missed something or did something wrong. I think we should merge this one quickly because the renaming will probably cause merge conflicts for any other branches on `models` ... I mean `modeling`.