Support OpenAPI 3.0.0

[jbowes]

Add support for generating code from OpenAPI 3.0.0 documents.

The latest version of OpenAPI should be the default internal representation of documents, so this change should look something like:

• add a new spec.go file to the openapi package, defining the document structure, and how to parse it
• teach the openapi/v2 package (or a new package) how to convert from v2 to v3
• teach the openapi package how to detect document types, and use the appropriate parser
• convert the rest of the code to reference types from openapi instead of openapi/v2.