Fix exit-status codes for Guard

[Maher4Ever]

While using Guard I always noticed that it exits with status code "1" even when using the interactor to stop it (My terminal shows failure status codes).
It would be nice to let Guard exit with the right status codes because it is a tool used by a lot of developers.

I've changed the way Guard exits in this commit. Guard doesn't exit the program no more when calling Guard.stop. This has the advantage of being able to use Guard.stop programmatically knowing it won't kill the process.

I felt that the interactor should be the one to exit Guard's process, so now it will call Kernel::exit after it stops all running guards.

When there is an interrupt, Guard will still stop all guards, but now it will use Kernel::abort to return a failure status-code.

The commits will apply nicely and the specs pass: http://travis-ci.org/#!/Maher4Ever/guard

[rymai]

Cool thanks! 🎁

[thibaudgg]

Great, thanks!

[thibaudgg]

@Maher4Ever this is ready to be merged?

[Maher4Ever]

Yes it is. Just merge the last commit though... the rest is already in Guard's master branch.