Switch to Scala 0.3.0 + async connection pool

Quite some changes here:

* Used official `ScalaBusMod` trait now, since it made it into the scala language module.
* AsyncConnectionPool now really is a pool and not creating and closing connections on `take` or `giveBack`.
* Added `FileHandlesTest` to check that it really isn't opening too many connections anymore - thanks @beders !
* Added `maxPoolSize` as configuration parameter.

This will get into master and hopefully be the 0.3.0 release as soon as CloudBees Jenkins is "back to normal".