exec java for interop with supervisors

We use runit to supervise marathon.  For those who wish to use bin/start
to launch marathon from a supervisor, it's necessary / beneficial for
the start script to exec the jvm, so that the supervisor is managing the
java process directly as a child (as opposed to bash->java).
Merge pull request #369 from cbuben/start-exec

exec java for interop with supervisors