Updated to register server.js as a command when "npm install -g" is run.

`npm install -g npm-offline` doesn't install a command at the moment. So, `npm-offline` doesn't do anything.

I added a bin entry in package.json so that `npm install -g` creates a symlink to server.js so that the `npm-offline` starts the server. It seems to work.

This is a great project, BTW! I've really needed this for the train. I've just been copying and manually updating package.json.