Add spark.executor.memory to differentiate executor memory from spark-shell memory.
Restore SPARK_MEM in executorEnvs.
Fix SPARK_MEM in ExecutorRunner.
Merge branch 'master' into sparkmem
Move executorMemory up into SchedulerBackend.
Fix dangling old variable names.
Merge pull request #379 from stephenh/sparkmem

Add spark.executor.memory to differentiate executor memory from spark-shell