resolved accelerationChange bug

Math.abs only accepts one argument. Before the difference wasn’t
calculated. Now it is.
we don't need getAccelerationSnapshot.

Because watchAcceleration is supported on all platforms us it direct
instead
remove stopWatch and setTimeout

call of assessCurrentAcceleration depends now on the frequency set in
options of watchAcceleration.
set previousAcceleration every time this function is called
possible to change sensitivity now

Set sensitivity when startWatch is called