User can add an image to auction

https://trello.com/c/rPefPOmu/13-as-a-user-i-can-add-images-to-posts

![image](https://cloud.githubusercontent.com/assets/8681076/4999535/212e7652-69b1-11e4-80a1-5afa729b4b5f.png)

We don't want to permit post_id or user_id; those are filled in via post.images.new and .merge(user_id: current_user.id)