SPARK-1126.  spark-app preliminary

This is a starting version of the spark-app script for running compiled binaries against Spark.  It still needs tests and some polish.  The only testing I've done so far has been using it to launch jobs in yarn-standalone mode against a pseudo-distributed cluster.

This leaves out the changes required for launching python scripts.  I think it might be best to save those for another JIRA/PR (while keeping to the design so that they won't require backwards-incompatible changes).