Bmm users can send and receive messages

https://trello.com/c/aoS4oDM7/12-as-a-user-i-can-send-and-receive-messages-from-other-users

![screenshot from 2014-11-24 15 01 43](https://cloud.githubusercontent.com/assets/8154429/5172097/ec89253e-73ea-11e4-8930-d44981ab8787.png)

I would move the `.order` into the `between` method, since I assume you always want to order messages between people this way.