Fix integration tests to actually run and work.
Add hook to force the sbt server to die.

* Add hook to server to make sure it is killed
* Add an isClosed method to the sbt client so we can
  have a chance of not doing something with a dead socket.
Cleanups from review.
Merge pull request #120 from sbt/wip/kill-command

new command to kill the sbt server directly, and some test cleanups.