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/23801/5142763/545ee888-717f-11e4-9a3e-2740bb48fcfd.png)

/cc @fofr I suspect that the SASS could be a bit drier?