Implemented non-blocking run mode

* Used this to implement a dev-mode scripted test.
* Reproduced #2905, assertions commented out to prevent failing.
@pvlugter, not sure if either of these methods are used directly by activator, do you know?  This stuff was quite messy, a lot non idiomatic sbt code, I've fixed it up, but I wasn't sure if there was a reason it was like that, or if it was just because it was written by people who weren't sure what they were doing.