initial support in MotionManager for accelerometer mode

This is a work in progress.  Adds an option passed in via enable() to only use accelerometer, if gyro is not available also does this.  Still to address:

* how to expose the 'matrix' that we build via getAcceleration(), which is in turn becomes the modelview (see MotionBasicApp.cpp, line 36)
* how to reduce the jitter