add customizing snapping animate duration property

[liuxuan30]

add customizing snapping animate duration property, so whoever uses definers can define the snapping animate duration. default is 0.15 secs

 *  Determines the animation duration while snapping
 */
@property (nonatomic, assign) CGFloat snappingAnimateDuration;

/**

[bryankeller]

I'll take a closer look this week, but I think this will be a good change. Thank you!

[liuxuan30]

Sure, it is a simple change. I need to snap my navi bar slower in my app. Not sure if other people want it