the feature for "instanceof" checking

[finscn]

Hi , @phoboslab ,

In Ejecjta , the below "instanceof" checking can't pass :

img instanceof HTMLImageElement 
canvas instanceof HTMLCanvasElement
video instanceof HTMLVideoElement
audio instanceof HTMLAudioElement

In PIXI.js v4 , the "instanceof" checking is very useful , I hope Ejecta could support it.

~~I add this feature in my fork version ~~
finscn@5f80a03

I hope it could help you.

There are too many differences between my fork version and origin ejecta , So I don't give a PR.
And I found that you have never accept my PR , maybe there are too many differences between our code spec. & style , So I think my PR is useless for you.

[finscn]

I found an easy way to implement this feature , just add this code to Ejecta.js

(function(){

    var element = document.createElement("img");
    HTMLImageElement = function(){};
    HTMLImageElement.prototype = element.__proto__;

    var element = document.createElement("canvas");
    HTMLCanvasElement = function(){};
    HTMLCanvasElement.prototype = element.__proto__;

    var element = document.createElement("video");
    HTMLVideoElement = function(){};
    HTMLVideoElement.prototype = element.__proto__;

    var element = document.createElement("audio");
    HTMLAudioElement = function(){};
    HTMLAudioElement.prototype = element.__proto__;

})();

Notice : the code must be after "window.document = { ... }" in Ejecta.js.