http/https proxy supporting

[almuntmd]

example of setting a http/https proxy:

        var conf = config.Config{
		AppId:               opts.AppId,
		AppSecret:           opts.AppSecret,
		AuthUrl:             opts.AuthUrl,
		PushUrl:             opts.PushUrl,
		HttpProxyUrl:        opts.HttpProxyUrl,
		HttpProxyCACertPath: opts.ProxyCACertPath,
		MaxRetryTimes:       1,
		RetryInterval:       0,
	}

	client, err := core.NewHttpClient(&conf)