Mtk checkboxes

Trello card: https://trello.com/c/4alh0io5/8-as-a-user-i-can-categorize-a-post-as-being-bikes-boats-etc

![post_categories](https://cloud.githubusercontent.com/assets/4560985/4954750/686e8acc-668f-11e4-9fa7-7775871d516a.gif)