Asynchronously call sys.exit() to avoid deadlock due to the JVM shutdown hooks.
Switch to scalatest

Using JUnit was sometimes awkward because there are no Scala-ish matchers.
Scalatest is mature enough now to use it here.
Purge the queue when scaling down apps. Fixes #254

Also improve logging.
Add a test for scale down
Merge pull request #268 from mesosphere/scalatest

Switch to scalatest
Switch everything to log4j
Bump chaos version
Merge pull request #267 from mesosphere/fix-254

Purge the queue when scaling down apps. Fixes #254
Merge branch 'pr/261'

Conflicts:
	src/main/scala/mesosphere/marathon/MarathonScheduler.scala