hopy you can add pan view ability

[a873054267]

same as the #150 ;
it's not easy to use ctrl +left click to pan the view,especially that need to adjust the points accurately;
I had try to modify the pan function fired by mouse middle down and move;but it's still not correct;
the core code is:
OrbitControls.js
this.mouseButtons = { ORBIT: THREE.MOUSE.LEFT, // ZOOM: THREE.MOUSE.MIDDLE, DOWNPAN: THREE.MOUSE.MIDDLE, PAN: THREE.MOUSE.RIGHT };
acctually,I had try to use left drag to control pan which is not effect;