Handle HATEOAS-style Links

[mmalone]

There's no easy way to define a Link field that corresponds to a URL (or some link structure) in a message from a remote server. The current fields.Link implementation allows you to construct a link programmatically to a remote object of a particular type and return that object on attribute access, but doesn't include that object (or a link to it) in the serialized dictionary after calling to_dict(), which is not ideal. A quick fix would be to serialize the entire linked object, but that's also problematic since it would force us to follow all links during serialization.