Pitch computations for accelerometer 

Code currently uses `Math.atan` which accepts only 1 argument and will silently drop the second. What we *want* is `Math.atan2` which returns the [arctangent of the quotient](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/atan2). See [here](http://www.freescale.com/files/sensors/doc/app_note/AN3461.pdf) for a primer on the math involved.