Dispose NotImplementedException Support

Here's an idea. Treat a dispose method with just `throw new System.NotImplementedException();` as empty, and don't error out.

The reason being a lot of code tools implement method stubs as just `throw new System.NotImplementedException();` so it would be nice if Janitor just dealt with the stub as is instead of me having to go and remove the code.

:+1: for laziness.