Fix start and stop scripts for Play 2.2.0

The dist task of Play 2.2.0 no longer creates target/start, instead it creates a script in target/universal/stage/bin. Also it puts the RUNNING_PID file in target/universal/stage when the generated script is used to start the app.

This patch fixes the script generation to work with Play 2.2.0, but does not address backward compatibility. I'm not quite sure what the best way of determining whether to use the new or the old layout. Maybe by grepping `project/plugins.sbt`, but that does seem a bit hacky and prone to breakage.