Users can comment on posts

https://trello.com/c/dDTterln/28-as-a-user-i-can-comment-on-an-auction

![selection_014](https://cloud.githubusercontent.com/assets/8248100/4995945/06048548-6992-11e4-8d89-2c65af048db6.png)
`null: false` for all foreign keys