adding difficulty badge and sorting to challenge selection

This fixes #133 by adding a colored badge with the label 'easy','moderate' or 'hard' and sorting them by easy first.

![screen shot 2014-04-07 at 6 03 05 pm](https://cloud.githubusercontent.com/assets/120452/2638568/42d26332-beb1-11e3-9eb3-f387938f3392.jpg)