Little improvements to mono-service land

I had to use a 3rd party Windows service which sadly makes MoMA bark when scanned, because it uses the ExitCode property. This lead me to remove the NIE, add a test, discover that Stop() was making my test fail, implement Stop(), add another test for the Stop() bit, and then after that, mono-service can happily use the ExitCode property to send the info to syslog!

Reviews welcome. Cheers