Pull accelerometer

In this pull request I replaced the rotation vector sensor to accelerometer and I made some other fix. In the getPitch and getRoll functions now will handle the overshoots of the max roll/pitch angle.

The reason of the sensors changes is that lot of device is not support the rotation vector sensor because it depend from Accelerometer, Gyroscope and Magnetometer (maybe except the gyroscope). https://source.android.com/devices/sensors/composite_sensors.html
Other side I don't see the point of support two sensor in the same time so I removed the fallback solution (commit: c821391)