Issue #210 - Add options to just store _id for refs [moved]

I recently had a scenario in a project of mine where I wanted to use the @Reference annotation.  Upon reading the mongoDB documentation warning against using DBRef and seeing how morphia implemented references, I wanted to contribute a feature that would allow you to specify using manual references instead of DBRef, without needing to manually fetch the entity using ObjectId.  To utilize manual referencing, you now can specify the option 'useDBRef' in the @Reference annotation, which defaults to true, so that this change is backwards compatible with previous behavior.