Add option/example to calibrate accelerometer

[TheLastGimbus]

Hi! Thanks for making this, finding a way to tilt-compensate compass was a pain, and this just works!

However, my tilt-compensation doesn't work perfectly, and I think that's because of accelerometer is not calibrated well enough - I'm getting X: -960 Y:-176 on perfectly flat surface.
It would be nice if there was just some .calibrateAccel() method to correct this