Allow primitive types for $parent metadata

[riannucci]

Example:

type MyThing struct {
  ID int `gae:"$id"`
  ParentID int `gae:"$parent,ParentKind"`
}

This would cause gae to synthesize MakeKey(aid, ns, "ParentKind", <ParentID>) for the $parent value.

The only thing I'm not super happy with is that this is very special-case to a 1-deep parent key, and until #41 is fixed, there's no ability to conveniently replicate this functionality with the MetaGetter interface.

edit: angle brackets

[riannucci]

@nodirt