Add event manager

[hak33m16]

Add an event manager with subscriber-receiver model as seen in entityx. Add base Event object

EventManager class should have:

• subscribe method that allows users to subscribe a function to a particular event
• emit method that allows users to emit events of any type

Only problem with the idea of subscribe method being something like:
subscribe<T extends Event>(event: T, handler: Function)
Is the whole binding issue in JS when it comes to classes. Idk if this is solved with ES6? e.g. https://github.com/hak33m16/ladder/blob/master/src/systems/RenderSystem.js#L44