Automatically upload scala api jar to cluster

It would be nice to automatically upload the Scala API jar to the cluster so that users don't need to do it manually.

There are a few drawbacks: that either forces us to maintain binary compatibility in future versions of the jar, or declare that the Scala API jar an applications uses must match the version of the Livy server.

Another option would be to somehow embed smarts into the Scala API code so that it auto-uploads its own jars; but that seems brittle and might have more drawbacks than the above.