Update for Spark 1.1 and 1.2 branch experiemental builds.
Merge remote-tracking branch 'upstream/master'
Add -x argument to install-spark to support automatically configuring cluster at startup for dedicate Spark job execution.
Update Spark README to include '-x' argument.
Added support for Spark 1.2.0 release with build 1.2.0.a.
Adjust for more headroom calculation on containers for drive rand executors.
Merge pull request #26 from christopherbozeman/master

Added Spark 1.2.0 release, update for Spark branch-1.1 and branch-1.2 experimental builds