Users can comment on posts

https://trello.com/c/dDTterln/28-as-a-user-i-can-comment-on-an-auction

![selection_014](https://cloud.githubusercontent.com/assets/8248100/4995945/06048548-6992-11e4-8d89-2c65af048db6.png)
remove the instance variables in the form partial and pass them in from the posts show page, so `post: @post, comment: @comment`