Implement Draft v4

[Colin-b]

closes #2
closes #3

Changed

• Update to latest RFC specification (draft v4) meaning:
  • affectedEndpoints is not sent anymore in case status is pass.
  • output is not sent anymore in case status is pass.
  • additional keys can now be provided.

Added

• healthpy.response_body function to retrieve a dict to return as JSON to the client.
• healthpy.response_status_code function to retrieve an HTTP status code to return to the client.
• healthpy.consul_response_status_code function to retrieve an HTTP status code to return to Consul.

Fixed

• affectedEndpoints is not sent anymore if not provided.

Deprecated

• Providing non mandatory parameters via positional arguments will be removed in the next major release.