Added property 'spark.executor.uri' for launching on Mesos without
requiring Spark to be installed. Using 'make_distribution.sh' a user
can put a Spark distribution at a URI supported by Mesos (e.g.,
'hdfs://...') and then set that when launching their job. Also added
SPARK_EXECUTOR_URI for the REPL.
Format cleanup.
Used 'uri.split('/').last' instead of 'new File(uri).getName()'.
Do not try and use 'scala' in 'run' from within a "release".
Merge pull request #749 from benh/spark-executor-uri

Added property 'spark.executor.uri' for launching on Mesos.