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 https://github.com/Jorricks/macos-notifications/pull/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]

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.