GPGREENMAIL-13 Fixed silently failing bug is stop()

The `GreenMail.stop()` method was simply wrong causing the server not to stop. This is a major issue for continuous builds as `grails test-app` effectively fails with `BindException`. The problem was three-fold. The `stopTimeout` variable was not defined. The bean's destroy method seems to fail silently in that case (although I am still puzzled how it compiles). In addition the original GreenMail package doesn't provide any service stopping method that only takes timeout. There is `Object` (probably meant to provide a way for passing additional attributes) and timeout, or just `Object`. Third, the `super.stop()` does exactly the same iteration thru services stopping them. So simple `super.stop()` is a proper fix for all of the above.