chore(): change vertxEventBusService definition to `provider()`

This commit changes nothing.
feat(mock): sockjs mock can emulate a login shake
feat(service): the service handles a login if defined

Introduces service.login(username, password): promise

* broadcast of vertx-eventbus.system.login.succeeded
* broadcast of vertx-eventbus.system.login.failed
feat(service): send/publish will additionally check login state

If `options.loginRequired` is set `true` and there is no active session,
the current message will be skipped.

Relates #14