Add endpoint to validate JSON of an SCS listing

This uses this implementation of JSON Schema: https://github.com/Julian/jsonschema
to validate JSON sent via a POST request to the API endpoint /g6/service/add against a basic schema for the SCS service JSON format currently exported by the SSP.

See this story: https://www.pivotaltracker.com/story/show/83934448
Yes, I agree it's not a very user friendly format but it is very explicit and very widely understood. The rendering of the date for people can be a frontend concern. The python package for jsonschema does support the dates.