Skip multiprocessing if no callback actions requested; Add sound param; Fix signal handling

[michelcrypt4d4mus]

Changes:

1. If there are no callbacks requested then there is no need for multiprocessing / listening / etc.
2. Fix the signal handling - the argument to sys.exit() is just the return code for the shell; it does not re-raise the SIGINT to call sys.exit(signal.SIGINT). Now it properly re-raises the trapped signal.
3. Add sound parameter as per the other PR Add sound param #25

In order to implement (1) I had to move the MacOSNotification class outside of the create_notification() method so it would not be redeclared every time create_notification() is called - see my comment.

Re: (2) I'm still not really convinced that trapping SIGINT in a package is the best idea and am kind of concerned about other unforeseen side effects but at least re-raising it seems to work as the user might expect.

Feel free to cherry pick and/or request changes.

[michelcrypt4d4mus]

@Jorricks why do you need both atexit() and custom SIGINT handling that do the exact same thing (call self.cleanup())? Seems like just atexit() should be sufficient to accomplish what you are going for?

[Jorricks]

I honestly do not recall.
I think we can indeed remove the catch_keyboard_interrupt but let's save that for another MR.

[michelcrypt4d4mus]

It turns out that because the MacOSNotification class isn't getting redefined with every call to create_notification() any more forking the initial NotificationProcess was unnecessary - you only need multithreading to listen for responses, not trigger the initial notification - so i removed NotificationProcess altogether on this PR.

[Jorricks]

First of all, sorry for being so slow to respond.
Second, I went over the MR and it looks very nice and ready to merge. Please rebase it/fix the conflicts and we can get it in.