Updated threex.domevent.js

Integrated ```threex.domevent``` with ```threex.Object3D``` as ```threex.domevent``` will not work as expected without it.

Updated a few function calls to ensure compatibility with newer versions of ThreeJS (Credit: http://learningthreejs.com/blog/2012/01/17/dom-events-in-3d-space/#comment-802169267)

Updated the read me at the top of the page to reference the need to use:

```THREE.Object3D._threexDomEvent.camera(CAMERA_VAR) ```

before setting up any events.