Transform fields on EmbeddedDocumentField

Fix in relation to #40 

```python
class C(Document):
    name = StringField()

class B(EmbeddedDocument):
    name = StringField()
    related = ReferenceField(C)

class A(Document):
    related = EmbeddedDocumentField(B)
```

Before change an `ObjectId(...) is not JSON serializable` would be generated from the `B.related` field when attempting to serialize `A`