Users can add images to their posts

https://trello.com/c/gYureyda/12-as-a-user-i-can-add-images-to-my-post

![image](https://cloud.githubusercontent.com/assets/8646429/4995926/df141336-6991-11e4-993f-37bc58e95619.png)

`null: false` for timestamps and foreign keys