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Merge branch 'master' of https://github.com/easemob/sdkexamples-ios

# By winterSleep
# Via winterSleep
* 'master' of https://github.com/easemob/sdkexamples-ios:
  添加被拒绝时的回调 添加好友请求时的reason 添加客户端在后台时, 收到消息的提示