Access `server`, and `server.address` after calling `app.createServer`

[ariporad]

This should be easy, but when calling ~~app.listen~~ app.createServer (an io can only be attached to one server), it would be nice if the returned server was also available (later on) as app.server (app.io already is). It would also be nice (as a convenience), to have app.address be an alias for server.address

Todo list:

• [ ] Calling ~~app.listen~~ app.createServer sets app.server
• [ ] Calling ~~app.listen~~ app.createServer sets 'app.address`
• [ ] Document