Added shorthand methods and 3rd Array args

I just realize that it might have been better to have a different order
in the array i.e. eventType first. However, I'd rather write [ true ]
than [ 'value', true, false ].
Some combinations won't be so common or allowed, so I think this will be fine.

To be able to have a .once() ability would be very nice for multiplayer games
or large indexes of fast changing data, where you wouldn't wan't to
update every ms, but rather manually every few seconds or so.

And an ability to specify the eventType is something I missed...

I've made sure that binding to props is not allowed, as it is bad practice. 
But it is nice to have the ability to retrieve some props from a remote firebase Index, for initial loading. For example: resuming a session after mounting a React component.