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)

No need for the `{ }` around this hash. You can still use implicit hash, `region: @region, post: @post` as it is the last argument. You can leave if you prefer, but not required and more in line with style to omit