add bounced back for slidemenu when slide not enough ( Misoperation )

add bounced back for slidemenu when slide not enough 
now must slide over 1/3width to open or close slidemenu

1.修复同一方向划动出现关闭slidemenu的问题
2.增加划动不足三分之一侧栏宽度时回弹的效果,避免误操作(平常轻轻一碰侧栏就开了)
Merge pull request #385 from SeekArt/master

add bounced back for slidemenu when slide not enough ( Misoperation )