Expose flingBezier, bounceBezier, flingTruncatedBezier and decelerateBezier

This commit exposes the 4 bezier properties (flingBezier, bounceBezier, flingTruncatedBezier and decelerateBezier) as options.
It allows one to tweak the behavior of the scroller.

These were declared as local variables (var _kFlingBezier, ...) so I kept them instead of rewriting everything (_kFlingBezier => _instanceOptions.flingBezier).
Tell me if you'd prefer that I replace the local variables.