Switch to using `userAccelerometerEvents`

[dangilbert]

I see you're working on a newer version which detects events better, but just wanted to say in case you hadn't seen already, that I've been having better success using userAccelerometerEvents instead of accelerometerEvents. This already has the gravyity removed.

I would often find on certain devices that just picking up the phone would trigger the onPhoneShake, but with the userAccelerometerEvents it seems to be much more stable.