Challenges

 - Implemented CSS for challenges (same as for Objectives, just different color)
 - removed redundant CSS for Objectives / Key Points
 - Clarified & rearranged challenge in lesson 02.
 - added contrib instructions.
The instructions for building lessons are all stored in the `README.md` of http://github.com/swcarpentry/lesson-template - what we'd like in the README for particular lessons is material particular to that lesson (e.g., how to regenerate the database).