add ability to submit multiple jars for Driver

i created a issue:
https://issues.apache.org/jira/browse/SPARK-2301
when standalone-cluster mode, use "org.apache.spark.deploy.Client launch master mainClass jarUrl" to submit driver. jarUrl doesnot support two jar,
example: org.apache.spark.deploy.Client launch master mainClass one.jar,two.jar .
so this PR can support jarUrl has more than two jars.

Also, can't you just keep the existing `downloadUserJar`, and introduce a new method called `downloadUserJars` that calls the old one after splitting `driverDesc.jarUrl`?