Give the choice to create a copy or dbref from a document in a dynamic Field

### Use a copy if value is a document 
myField = DynamicField(use_dbref=False)

### Use a DBRef if value is a Document
myField = DynamicField()
#or
myField = DynamicField(use_dbref=True)


### Example:


    class Doc3(EmbeddedDocument):
        field_2 = StringField()
    
    class Doc2(Document):
        field_1 = StringField()
    
    class Doc(Document):
        my_id = IntField(required=True, unique=True, primary_key=True)
        embed_me = DynamicField()
        embed_me_nodbref = DynamicField(use_dbref=False)
    
        embed_me_ed = DynamicField()
        embed_me_ed_nodbref = DynamicField(use_dbref=False)
    
        field_x = StringField(db_field='x')
    
    Doc.drop_collection()
    Doc2.drop_collection()
    
    doc3 = Doc3(field_2="hello2")
    doc2 = Doc2(field_1="hello")
    doc = Doc(my_id=1, 
        embed_me=doc2, 
        embed_me_nodbref=doc2, 
        embed_me_ed=doc3, 
        embed_me_ed_nodbref=doc3, 
        field_x="x")
    
    doc2.save()
    doc.save()
    
    doc = Doc.objects.get()
    
    print doc.embed_me_nodbref.field_1 == doc.embed_me.field_1 # True : hello = hello
    
    doc.update(set__embed_me_nodbref__field_1 = "world")
    doc.reload()
    
    print doc.embed_me_nodbref.field_1 == doc.embed_me.field_1 # False : world = hello
    
    print doc.embed_me_ed_nodbref.field_2 == doc.embed_me_ed.field_2# True hello2 = hello2