Adding adaptive tone-mapping postprocessing pass and example

I've written a postprocessing pass that tone maps the scene (using the standard Reinhard method) based on the average luminance of the scene. This works best when using an HDR render target (i.e. a type of THREE.FloatType).

The provided example shows a comparison between static tone-mapping and adaptive tone-mapping with both LDR and HDR render targets. HDR and dynamic tone-mapping allows you to render scenes with high-contrast lighting conditions (such as the day and night sides of the Earth as in the example).