Fix for passing quoted args with spaces to shell()

Before, passing a command with quoted arguments that contained spaces to 
slate.shell() produced an error. This works now.

I'm not used to ObjectiveC, so please be lenient. I had quite some
problems getting the tests to run and then, in my test, result also
contained "GC: forcing GC OFF because OBJC_DISABLE_GC is set" which is
why I used a regular expression to test the result, which isn't too
clean. Maybe you know a better way to test this.