Arealight for MeshPhongMaterial.

There was always an AreaLight class but it was only used in the deferred renderer examples.  This is an implementation that works with MeshPhongMaterial.

It isn't perfect and it is based on http://stackoverflow.com/questions/17021264/improved-area-lighting-in-webgl-threejs

This PR is not complete.

Includes my merge parts of these two other PRs: https://github.com/mrdoob/three.js/pull/5785 https://github.com/mrdoob/three.js/pull/5788