Expose and document events for better integration.

[patricknelson]

As noted in my previous issue #89, I also have an issue with my existing controller having it's own laser which coincidentally exactly overlaps with the laser created by this library. Currently there's no way to easily determine how the user is currently interacting with dat.guiVR except maybe by checking guiInput.visible. It would be great if the API were extended to also expose and document the current events on input.events (hidden internally), such as onControllerHeld and grabbed (etc) and also maybe new events such as pointing or similar.