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)

The image is getting the associated post through the `"image/form", post: @post, image: @image`