Merge the old sbt-launch-lib.bash with the new sbt-launcher jar downloading logic.

This allows developers to pass options (such as -D) to sbt.  I also modified the SparkBuild to ensure spark specific properties are propagated to forked test JVMs.

Author: Michael Armbrust <michael@databricks.com>

Closes #14 from marmbrus/sbtScripts and squashes the following commits:

c008b18 [Michael Armbrust] Merge the old sbt-launch-lib.bash with the new sbt-launcher jar downloading logic.