Set spark.executor.uri from environment variable (needed by Mesos)

The Mesos backend uses this property when setting up a slave process.  It is similarly set in the Scala repl (org.apache.spark.repl.SparkILoop), but I couldn't find any analogous for pyspark.

Author: Ivan Wick <ivanwick+github@gmail.com>

This patch had conflicts when merged, resolved by
Committer: Matei Zaharia <matei@databricks.com>

Closes #311 from ivanwick/master and squashes the following commits:

da0c3e4 [Ivan Wick] Set spark.executor.uri from environment variable (needed by Mesos)
Add Spark v0.9.1 to ec2 launch script and use it as the default

Mainly ported from branch-0.9.

Author: Harvey Feng <hyfeng224@gmail.com>

Closes #385 from harveyfeng/0.9.1-ec2 and squashes the following commits:

769ac2f [Harvey Feng] Add Spark v0.9.1 to ec2 launch script and use it as the default