Add duration to posts, add posts#index for users

![image](https://cloud.githubusercontent.com/assets/8669803/5018533/8b33ceec-6a85-11e4-9272-93d916f31f39.png)

This should just be in the view somewhere, or a partial. Usually helper methods just return a link or a little 20-character string. Generating a whole form is too much.