Splitting off from #1178.

@smarterclayton For the API overhaul. @dchen1107 may also be interested.

Goals:

• uniform structure across all API objects
• clean separation of input and output values

Currently, our API objects contain:

• (all, even lists which is questionable) JSONBase, which includes Kind, ID, CreationTimestamp, SelfLink, ResourceVersion, and APIVersion
• (Pod, ReplicationController, Service) Labels
• (Pod, ReplicationController) DesiredState
• (Pod only) CurrentState

In JSONBase, only ID is explicitly provided by the client and only ResourceVersion is mutable.

In Pod, DesiredState and CurrentState use the same schema, mixing input and output fields (Manifest, RestartPolicy, Host, HostIP, PodIP, Info).

Service lacks DesiredState and CurrentState.

ReplicationController lacks CurrentState (though that may change with #736).

Proposals:

1. Nest JSONBase in a named field, such as Metadata, to more coarsely bucket them within the objects.
2. Move Labels to JSONBase, so that they're in the same position in every object, together with other generic object metadata.
3. Add CurrentState to all objects that are lacking it.
4. Use different representations for desired and current state in Pod, appropriately divide the fields between them, and merge ContainerManifest into the new PodDesiredState type.