Add image cutout module to mast

Currently the cutout image services are not covered by the mast module, and already existing services (e.g. http://ps1images.stsci.edu/cgi-bin/ps1cutouts) are not planned to be covered in the near future (https://github.com/astropy/astroquery/issues/896#issuecomment-381201394).

So this new addition may be a good hack day project for a new contributor.

Start by reading the existing MAST module and the discussion linked from issue #896, then inspect how MAST services are currently exposed. Identify the cutout image services that should be covered and define completion as working MAST cutout support with appropriate verification against those services.

Written by the indexing model from the issue text.