Discussion formatting improvements

![screenshot from 2014-06-25 16 38 08](https://cloud.githubusercontent.com/assets/383630/3387487/c79fcf68-fc7e-11e3-85b8-6ad19d7cd7b7.png)

Please avoid using `$gutter` (I will deprecate it today).

Also: we use gutters for horizontal spacing. For vertical spacing, use `$gs-baseline`.