_dynamic field updates - fixed pickling and creation order

Dynamic fields are ordered based on creation and stored in _fields_ordered (#396)
Fixed pickling dynamic documents `_dynamic_fields` (#387)

tests/fixtures.py

@@ -17,6 +17,14 @@ class PickleTest(Document):
     photo = FileField()
 
 
+class PickleDyanmicEmbedded(DynamicEmbeddedDocument):
+    date = DateTimeField(default=datetime.now)
+
+
+class PickleDynamicTest(DynamicDocument):
+    number = IntField()
+
+
 class PickleSignalsTest(Document):
     number = IntField()
     string = StringField(choices=(('One', '1'), ('Two', '2')))