Preliminary Addition of MultiDark Simulation Database Query Tool

This it my first pull request of the addition of simulation datbase query tools to astroquery. As it stands, things are in rough shape, but I welcome all comments, concerns, and tips. Thanks!
It would be better to use `astropy.utils.data.download_file` or `astropy.utils.data.get_readable_fileobj` here because of the caching utilities they include