Fix replyTo wiring through server.
Add failing test for ui interaction that we can fix to work.
Fix integration tests such that the forked sbt has a completely clean environment, using new server hooks and hackery.
Fix interaction wiring so it correctly flows through the system.

* Fix a few replyTo invalid usages.
* Fix IPC protocol to include replyTo serial everywhere.
* Update test to correctly end.
* Remove non-longer-necessary debugging output.
* Make sure Responses extend Response to we hit the correct pattern match.
Add basic interaction from ui plugin to simple sbt terminal
Merge pull request #66 from sbt/wip/fix-interaction-replyTo

Fix interaction to be completely wired.