Add support for arbitrary properties

[glennrfisher]

Several models for the Conversation service can contain arbitrary nested properties. We need to make sure that (1) these properties are sufficiently captured and sent back to the service, and (2) these properties are accessible to users in a convenient, easy-to-use way.

For example, the context model can include additional information based on the client's code. A user can set "pizza_toppings": ["ketchup", "ham", "onion"]. This should be accessible to users of the SDK and also propagated back-and-forth to the server.

[glennrfisher]

This issue is complete in the swift-3.0-remove-dependencies branch. We should document this feature in the readme, however, so I opened to #511 as well.