Rename property, move property, fix typo.

bddr_doubleTapZoomsToMinimumZoomScaleAtMaximumZoom is too long but expressional.
maxmimumScale is actually maximumZoomScale…
Move bddr_zoomScaleStepFactor declaration down, after all the other properties that use it.
Add note in bddr_zoomScaleStepFactor's documentation about the usage for one-finger-zooming.
Add one-finger-zoom-bouncing, improve one-finger-zoom zoom scale calculation.

bouncesZoom will be respected when one-finger-zooming.
Calculating the new zoom scale still seems wrong…