Extend ScrollView to Support PULLDWON and PULLUP,as well as Zoon its Header
扩展ScrollView，支持上拉下拉，且可以Zoom其头部。。
效果图如下：
#Hint Header布局需要内置在自定义的XML文件的Layout里。
The header is the first child In the Linearlayout whose parent view is ZoomScrollview
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