Only check id as part of embedsMany relation
More tests for embedsMany with persistent model
Introduce embedsMany persistent: true option

When set, instead of only embedding the model (on creation) it will be
persisted first, and subsequently embedded.
Implemented persistent: true option for embedsOne
Allow embedsOne to use auto-generated id (from connector)

If the connector has a `generateId` method - like Transient - it can be
used to set the id (when the property has been set to `generated:
true`).
Add test case for Numeric ids (with optional forceId)
Merge pull request #295 from fabien/fix/embeds-many

Various improvements to embedded relations