Add maintainers to Listing

See https://github.com/EclipseFdn/marketplace-rest-api-specs/issues/27

Please see: https://bugs.eclipse.org/bugs/show_bug.cgi?id=551901

it was decided that the API should return information about the organization and the author. MPC will display the organization and if that information is missing, they will print the authors name.

This has been completed