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
If we could get the SSP to output these as [RFC3339](http://tools.ietf.org/html/rfc3339) (basically a slightly stricter [ISO-8601](http://en.wikipedia.org/wiki/ISO_8601)) we could use the JSONSchema [date-time format](http://spacetelescope.github.io/understanding-json-schema/reference/string.html#built-in-formats).