Json schema

[andersoncardoso]

Refers to #8 and #14

• Add generic abstraction for json-schema validations. Handle validations, can be used with multiple schemas, and parse errors to be more meaningful.
• Integrate json-schema validation on the api (refactor to: remove duplicated validations from grape, use service objects, return errors with schema ref urls)
• EnvelopeBuilder service obj to handle multi validations on envelopes
• json-schema definitions for:
  • envelope
  • delete_envelope
  • learning_registry
• api endpoint to return schemas