added json-schema based compositin examples

There are two samples now.  04-models-with-subtypes.json which shows the json-schema conventions for modeling composition with the `anyOf` keyword.  The file 05-models-with-subtypes.json shows a more compact representation which is still valid with json schema.  There are fewer `$ref`s, and definitions are more in-line.

The spec will support json schema for representing the models so there will be different techniques to represent payloads.  Instead of being overly perscriptive in the spec, we will support json-schema and provide examples of different ways to model payloads, including with composition, alternate input/output models, etc.