Added Compass

Added a simple(but not cheezy) compass. The compass is hidden if a Magnetometer sensor is not detected.
I'm totally open to any input you may have about changes or different ways to implement this... 

![9ax8fzc](https://cloud.githubusercontent.com/assets/5514582/3488168/2ba9a5fc-04d0-11e4-8934-718dee9e3512.png)

Here is a GIF of it in action.
http://www.gfycat.com/SorrowfulAccomplishedEquestrian