Exit code is now passed along to bin stub

* Without these return statements bin/mandrel-runner always returns None
* For completeness, I also added a return statement to the launch_script method
** However, I haven't dived into the script code (just the runner)
** It may actually not be needed
* All 84 tests seem to pass
Merge remote-tracking branch 'will/master'
Merge branch 'master' into release
version 0.2.0

mandrel-runner using return value as the exit code is a change
to the interface, though probably a minor one for most uses.