embedsMany (subdocs) and referencesMany (array of ids)

Follow up to PR #194 - it should now cover most of the scenarios. I think the polymorphic embedsMany is particularly interesting, as it allows cross-model ordered relations. Docs will follow soon.

What's important though: LB needs to be updated to incorporate the remoting methods for this.

See also: http://mongoosejs.com/docs/subdocs.html and
http://mongoid.org/en/mongoid/docs/relations.html#has_many