Users and admins can cancel an auction

https://trello.com/c/MXqrE83d/23-as-a-seller-i-can-cancel-an-auction

https://trello.com/c/9ZWydogb/16-as-an-admin-i-can-cancel-auctions-delete

![image](https://cloud.githubusercontent.com/assets/8681076/4994574/454b683e-6988-11e4-913f-1a0a5cca1fca.png)


Got carried away and did a second trello card at the same time.
How about adding this to the `User` model like this:

```ruby
class User
  def can_modify?(post)
    admin? || post.id == id
  end
end
```