Ability to show active breakpoint in corner of viewport

A little bonus feature that I've found quite useful for debugging and cross-device testing: Flip a switch to show the active breakpoint in the top right corner of the viewport.

![show-breakpoints](https://f.cloud.github.com/assets/121186/1596407/3cfe0596-52fb-11e3-8d92-6454c2fef2b4.gif)