Users can mark posts as spam

https://trello.com/c/o0uBiAEc/22-as-a-user-i-can-mark-auctions-as-spam
![selection_015](https://cloud.githubusercontent.com/assets/8248100/4999734/f3033ab8-69b2-11e4-9079-5b77b86b3c22.png)
![selection_016](https://cloud.githubusercontent.com/assets/8248100/4999748/11a125d4-69b3-11e4-8398-fd43894585e0.png)

We don't need post.toggle(:spam) as spam is being set right after with the update method