Users can add images in their posts

https://trello.com/c/mgmyNR6K/15-as-a-user-i-can-attach-an-image-to-a-post
![screenshot from 2014-11-05 14 41 07](https://cloud.githubusercontent.com/assets/8154429/4924486/b60d916e-6523-11e4-9058-5061ab308999.png)