function keydown( event ) {

		if ( _this.enabled === false ) return;

		window.removeEventListener( 'keydown', keydown );

		_prevState = _state;

		if ( _state !== STATE.NONE ) {

			return;

		} else if ( event.keyCode === _this.keys[ STATE.ROTATE ] && !_this.noRotate ) {

			_state = STATE.ROTATE;

		} else if ( event.keyCode === _this.keys[ STATE.ZOOM ] && !_this.noZoom ) {

			_state = STATE.ZOOM;

		} else if ( event.keyCode === _this.keys[ STATE.PAN ] && !_this.noPan ) {

			_state = STATE.PAN;

		}

	}
@author Eberhard Graether / http://egraether.com/
@author Mark Lundin 	/ http://mark-lundin.com