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'd seen that JSONSchema supports that.  I wrote this one to match the format that currently comes out of the SSP, but as I'm re-doing it it might be worth changing the format for the export (I think we still need the pretty version that gets displayed to users though - that RFC is quite ugly IMHO!).
I also wondered about the note: "JSON Schema implementations are not required to implement this part of the specification, and many of them do not." - even if the implementation we pick to use now does support it, any change of library in future might break this validation.