Support loading of mixins

See https://github.com/strongloop/loopback-datasource-juggler/pull/201
I am taking it back. One of my earlier comments mentions that a mixin can be either a function or an object. If I understand the intention correctly, the latter allows the developer to specify a mixin like an object with some properties and these properties are then added to the target model.

I don't know enough about the implementation in juggler, is it necessary that this object has to be an instance of loopback Model?