Add action method overloads that take other resources as parameters

[david-at-aws]

It'd be slick to be able to pass resources as inputs to actions on other resources, and describe in the model how to map identifiers and/or data from the parameter onto the action request. For example:

Group badDudes = iam.getGroup("BadDudes");
User david = iam.getUser("David");

badDudes.addUser(david);

Comment here if you'd like to see this sooner rather than later!