Restructure astroquery

I propose to restructure the astroquery files like this:

* `astroquery/service/service.py` is renamed to `astroquery/service/core.py`
* Tests go in `astroquery/service/tests/`, not all in `astroquery/tests`.
* Imports are all relative, even from tests (like in astropy)
* Names are not (partially) repeated, e.g. `astroquery/simbad/sim_parameters.py` is renamed `astroquery/simbad/parameters.py`.

@keflavich @astrofrog What do you think?

I can do it now, but I don't know how rebasing will go for existing PRs. Do you want to merge  e.g. #68 and #70 now and then we just keep working on them in new PRs after the restructure?