Rename e --> part in MessageEncoderMongoDB.replace_id
Add tests for MessageDecoderMongoDB

test_hook_only_object_id

  The hook method only affects the "object_id" key

test_hook_id_is_objectid

  The hook method makes an bson.ObjectId from the "object_id" key
  and stores it into "_id"

test_after_hook_no_more_object_id

  The hook method removes any "object_id" key.