深入理解Google Cast（二）开发一个支持Google Cast的sender APP_castbuttonfactory

前言
０．Activities需要继承自ActionBarActivity或FragmentAcitivy
０． cast button需要在所有Actvity中存在，允许用户随时连接，随时断开．

初始化CastContext

首先实现OptionsProvider接口来获得CastOptions，再用CastOptions初始化CastContext
receiver application ID：用于过滤Cast Devices搜索结果并且在Cast会话开始时启动receiver　application．
When you develop your own Cast-enabled app, you have to register as a Cast developer and then obtain an application ID for your app.

import com.google.android.gms.cast.framework.CastOptions