setParent option to prevent adding child ids to parent documents in hasMany

setParent option allows a `hasMany` association to not save its child ids on itself.

- mongo-relation queries children by parent's id on child as a foreign key
- Mongoose populate on a Model will not populate children
- mongoose populate on a document will return an error (use find)

- I believe we can bring `doc.populate` back eventually. Probably after we've moved to mongoose-long & our own types (instead of adding to MongooseArray.prototype)

We should discuss keeping a separate branch until the breaking changes are resolved.