WIP #41 Assign agent to conversation

The first agent to view a conversation is automatically assigned to it, which is also shown in the conversation detail view like this:

![image](https://f.cloud.github.com/assets/228734/1946503/d1d2de30-800a-11e3-8d2e-7fe7a66932e0.png)

Closes https://assemblymade.com/helpful/wips/41