added callback for renderer size
added callback for renderer size

window.innerWidth and window.innerHeight are only useful, if the canvas
is actually filling the whole screen. This is not always the case. E.g.
the game has a sidebar for ads, an user interface written in html/css at
the bottom, a navigation bar at the top, etc.

Example: The game has a chat at the bottom with a height of 200px, the
new code would look like this:

```var windowResize = new THREEx.WindowResize(aRenderer, aCamera,
function(){ return { width: window.innerWidth, height:
window.innerHeight - 200 } })```

The dimension callback can also be used to execute other functions on
window resize without manually adding another event listener.

jeromeetienne/threex.windowresize