add support for yarn-client mode

Hi,
	In my spark on yarn cluster, it takes about 20s to create sparkContext in yarn-client mode. While creating sparkContext, akka Future will get timeout error.So, I set the Future timeout to 40s in yarn-client mode, and also longer the spray's "request-timeout" to 40s.
	In yarn deployment, spark master is not a permanent process. No longer need to start sparkWebUi.
	I test codes on spark 1.0.2 and cdh5.1.0, spark-jobserver can work.
:)


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