Chen post edit delete

https://trello.com/c/nIkxgiNB/12-as-a-user-i-can-update-and-destroy-my-posts

edit post
![edit-post](https://cloud.githubusercontent.com/assets/3759510/4941660/1c248856-65e5-11e4-85fe-62ce26aada98.gif)

delete post
![delete-post](https://cloud.githubusercontent.com/assets/3759510/4941662/1fb5438e-65e5-11e4-9766-02be99bd7875.gif)

I think we can just pass the category, since the post is owned by the category.