Merge pull request #23 from jerryshao/multi-user

Add Spark multi-user support for standalone mode and Mesos

This PR add multi-user support for Spark both standalone mode and Mesos (coarse and fine grained ) mode, user can specify the user name who submit app through environment variable `SPARK_USER` or use default one. Executor will communicate with Hadoop using  specified user name.

Also I fixed one bug in JobLogger when different user wrote job log to specified folder which has no right file  permission.

I separate previous [PR750](https://github.com/mesos/spark/pull/750) into two PRs, in this PR I only solve multi-user support problem. I will try to solve security auth problem in subsequent PR because security auth is a complicated problem especially for Shark Server like long-run app (both Kerberos TGT and HDFS delegation token should be renewed or re-created through app's run time).