remove support for proxying to another file

instead it now exports `true` if it creates a new process and `false` otherwise. This means you don't need to have two executables which means it will be faster and in my opinion cleaner. This means you can't pass args like `--harmony` to your main executable but you can put them in the hash bang anyway. So if you want to always run with `--harmony` set you do `#!/usr/bin/env node --harmony`.

I think I would always prefer this API but if you think some people might prefer the current one I'm happy to maintain a fork.