EC2 script changes for spark 1.2.0

Changes to thunder-ec2 script for compatibility with the current Spark 1.2.0 rc1 build.

Main new functionality here is a -v/--version parameter to allow the Spark version to be specified, as in the Spark ec2 script on which this is based. The thunder-ec2 script will enforce a minimum requested Spark version of 1.1.0, and a maximum Spark version of whatever is installed locally in $SPARK_HOME. (So if you have 1.1.1 installed locally on the machine from which you are running thunder-ec2, you'll have the option of installing 1.1.0 or 1.1.1, but not 1.2.0.)

Resolves #62.