Introduce EventModule to simplify event monitoring.

[LazoYoung]

Expected usage of the module in script:

// event is instance of GameAreaEnterEvent.
EventModule.onPlayerEnterArea { event ->
    if (event.getTagName() == "buff") {
        event.getPlayer().addPotionEffect...
    }
}

// event is instance of NPCClickEvent from CitizensAPI.
EventModule.onPlayerClickNPC { event ->
    if (event.getNPC().getName() == "detective") {
        def player = event.getClicker()
        def itemStack = player.getItemInMainHand()
        if (itemStack.isSimilar(evidence_item)) {
            player.sendMessage("Bob should be blue.")
        }
    }
}