实现APK版本检查、下载更新及自动安装。
CircleProgress（https://github.com/lzyzsd/CircleProgress） android-async-http（https://github.com/loopj/android-async-http）
Copyright 2014 gonglei
Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
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