[BUILD] Add the ability to launch spark-shell from SBT.

Now you can quickly launch the spark-shell without building an assembly.  For quick development iteration run `build/sbt ~sparkShell` and calling exit will relaunch with any changes.

Author: Michael Armbrust <michael@databricks.com>

Closes #4438 from marmbrus/sparkShellSbt and squashes the following commits:

b4e44fe [Michael Armbrust] [BUILD] Add the ability to launch spark-shell from SBT.