Added magnetometer calibration to pilot plugin.

This change switches the HUD compass to use the AR.Drone magnetometer.

I'm not positive I'm using the correct data from the on-board compass, but it seems to be working.

This also adds a button in the upper right to tell the drone to perform the magnetometer calibration routine.  The button is only enabled when the drone is in the air, and pressing it causes the drone to spin in place 360 degrees.  This requires a recent node-ar-drone that supports client.calibrate.

I put this in the pilot plugin, but if you think it should go somewhere else, let me know.

(I also untabified files I edited for this commit that had mixed spaces/tabs.)
![screen shot 2013-05-28 at 6 46 53 pm](https://f.cloud.github.com/assets/52466/576397/db703170-c801-11e2-83dd-2015f4e3ccaa.png)