Support orthographic camera

[rdeits]

It would be nice to make orthographic projections easy to create and still controllable with the mouse.

The current workaround is: (in the Javascript console):

camera = new THREE.OrthographicCamera(-2, 2, 2, -2, -2, 2);

and then to use camera.rotateY(Math.PI / 2) to rotate the camera.

cc @pangtao22