Add hint gutter behaviour

Adds the option to have a gutter hover behaviour to display hints.

![screen shot 2014-04-28 at 9 29 56 pm](https://cloud.githubusercontent.com/assets/185091/2816362/815d3b96-cec8-11e3-8b5a-92e469059ac5.png)


![screen shot 2014-04-28 at 9 28 43 pm](https://cloud.githubusercontent.com/assets/185091/2816355/57d205e0-cec8-11e3-8365-471b1e70ff78.png)