Extract a "main" function which sets up signal interrupts

[jnicklas]

See https://github.com/thefrontside/effection.js/issues/68

[cowboyd]

This is really nice also, because it concentrates our error handling around the entire start script. With the new version of effection this was a gotcha with the new behavior because the old behavior allowed you to catch exceptions that happened in async forks. But because that's no longer allowed (for good reason) then we need a "container" operation to be a single error boundary. We get this for free with @effection/node