Add a callout component

This is cribbed from the [callouts used in the Bootstrap documentation](http://getbootstrap.com/components/#glyphicons-how-to-use).

![image](https://cloud.githubusercontent.com/assets/23801/5143027/0c28402e-7183-11e4-84c0-f2e9d051395e.png)

/cc @fofr I suspect that the SASS could be a bit drier?