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
Unbounded catches are generally considered a bad idea in Python. You can potentially catch Python internal errors that can leave the program is an undefined state.