Add duration to posts, add posts#index for users

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

Ah, OK. "value=" is unnecessary, though - it's setting a local variable called `value`, then passing that local variable as the second parameter to the method. It works, but for wacky reasons.