Batch & Background: fast break!

In continuation to #57 and #42, I propose here to:

* fix `-batch` option
* auto-detect if running in background (`&`) and act accordingly, so that sbt execution is not interrupted.

Here a few more details about addressing #42:
Like @paulp in previous commit (f8a0213), I don't propose any fix to wait and catch exit code of sbt run (running in background already means some loss of interest on this value, I guess). For the records, I attempted tricks with things like `wait $!`, `sleep`, `disown`, `nohup`, ... but both (parent script and sbt launcher) processes always get **stopped** when such builtins are called. I shortly gave up to investigate on this, since I'm personally only interested in `-batch` mode. @kim0 could you please confirm that it meets your needs?

Note that