added set of options

Alright, let's see the changes.

The third parameter of THREEx.WindowResize has been changed to an
options object.

The options object has a width and height getter, aswell as several
scaling options. Imagine someone loading your game on a widescreen tv.
How many fps will he get?

Now you can define a maxWidth and maxHeight property + a scaling method
(3d transform, 2d transform, width/height scale, no scale) and the
window resizer will scale the renderer for you. This most likely can be
improved later on.

Another change is the before and the after callback. Sometimes you want
to do something after or before the renderer is resized. This will allow
you to keep all your resize handlers perfectly organized.

To change a setting on the fly, you can simply say
```javascript
var windowResize = new THREEx.WindowResize(renderer, camera);
windowResize.option("maxWidth", 1234);
```

This can be useful for ingame settings where players can decide to make
their viewport smaller to increase their fps and gaming experience.

Opinions?

```javascript
	var windowResize = new THREEx.WindowResize(renderer, camera, {
		width	  : function() { return $(window).innerWidth(); },
		height	  : function() { return $(window).innerHeight() - 200; },
		maxWidth  : 1024,
		maxHeight : 756,
		after	  : otherResizeHandlers,
		scale	  : '3d'
	});
	windowResize.trigger();
```

This is just a proposal. Should be discussed though!