New Exercise: Relative and percent error

I made an exercise for practicing relative and percent error. This is a response to [Trello Card #451](https://trello.com/card/relative-percent-error/4d87e664967a0775082939ab/451). 

I've tried to make the hints as helpful as possible and keep the experimental value close to the actual value to simulate realistic amounts of error.

![Screen Shot 2013-02-14 at 6 09 30 PM](https://f.cloud.github.com/assets/778344/159331/ca4cd1b6-7714-11e2-9175-5bae7bc4592c.png)