OrbitControls pan and multitouch support

Added pan for perspective and orthographic cameras.
Added multitouch control: 1 finger rotate, 2 zoom, 3 pan.
To make the control a (mostly) drop-in replacement for Trackball:
Added "target" (same as "center", which is deprecated)
Change userZoom to noZoom, etc.
Change userZoomSpeed to zoomSpeed, etc.
Removed unused functions and other cleanup.

Modified a few demos to use this control, where an orbit makes more sense than trackball. In other words, those with a ground plane, where there's a definite up direction.