Add badge for android-gems.com

[liaohuqiu]

Now you can add a badge to your README.md, it looks like this:

You can paste the following text to the README.md:

• Markdown

  [![Android Gems](http://www.android-gems.com/badge/saiwu-bigkoo/Android-AlertView.svg?branch=master)](http://www.android-gems.com/lib/saiwu-bigkoo/Android-AlertView)

• or HTML

  <a href="http://www.android-gems.com/lib/saiwu-bigkoo/Android-AlertView"><img src="http://www.android-gems.com/badge/saiwu-bigkoo/Android-AlertView.svg?branch=master"></a>