SPARK-1337: Application web UI garbage collects newest stages

Simple fix...

Author: Patrick Wendell <pwendell@gmail.com>

Closes #320 from pwendell/stage-clean-up and squashes the following commits:

29be62e [Patrick Wendell] SPARK-1337: Application web UI garbage collects newest stages instead old ones
SPARK-1350. Always use JAVA_HOME to run executor container JVMs.

Author: Sandy Ryza <sandy@cloudera.com>

Closes #313 from sryza/sandy-spark-1350 and squashes the following commits:

bb6d187 [Sandy Ryza] SPARK-1350. Always use JAVA_HOME to run executor container JVMs.
SPARK-1404: Always upgrade spark-env.sh vars to environment vars

This was broken when spark-env.sh was made idempotent, as the idempotence check is an environment variable, but the spark-env.sh variables may not have been.

Tested in zsh, bash, and sh.

Author: Aaron Davidson <aaron@databricks.com>

Closes #310 from aarondav/SPARK-1404 and squashes the following commits:

c3406a5 [Aaron Davidson] Add extra export in spark-shell
6a0e340 [Aaron Davidson] SPARK-1404: Always upgrade spark-env.sh vars to environment vars