Great idea! Tests would be great as I can't push the changes without the tests. Also since we are adding callbacks I think that there should be callbacks specific to each endpoint so this could be the "GlobalCallback" that runs for all endpoints, and then another that is added when you create an endpoint, for example

longpoll.create("/hello", subscribeCallback1); 
longpoll.create("/world", subscribeCallback2); 
longpoll.setGlobalCallback(globalSubscribeCallback); // or through initial config

I'll take care of the endpoint specific callbacks.

Thanks for making a pull request!