Nonrequired embedded resources

Based off my other PR, it would be nice to allow for embedded resources to sometimes not be required.
@olegpidsadnyi good point. I suppose it wasn't quite obvious enough to me that in order to have an non-required field not render in the serialized document that I should simply raise an `AttributeError`. Silly me. However, I do believe that the `required` parameter should be available for `Embedded` documents. No?