Allow additions to interpreter classpath

[mergeconflict]

There isn't really any way yet to demo anything from, let's say, twitter-util or finagle. Need to be able to pull external libraries into the interpreter's classpath to allow this.

[jnievelt]

console.classpath.value = classPathList.mkString(":")

[mergeconflict]

@jnievelt yeah, right now I'm doing settings.usejavacp.value = true. So actually, any dependency you add to the build.sbt file with compile or runtime scope will actually be present in our classpath. That might be good enough...