extend API

Methods:
bounceForward, bounceBackward for programmatic bouncing
Properties:
swipeThreshold (float, default 125.0)
swipeEscapeVelocity ( float, default 650.0) defines, when animated flip
is triggered
bounceRatio (float, default 0.3) bounce level when doing programmatic
bounce
rubberBandRatio (float, default 0.67) level to which first/last page
can be flipped