Unify Loaders

This pull request attempts to unify all loader syntax. 

Bonus:
- new ```onStart``` event in the ```LoadingManager```
- plumb the download manager and crossOrigin all the way down to the texture loaders

Remaining Inconsistencies:
- ```BinaryLoader``` and ```JSONLoader``` both return 2 parameters to their callback method, whereas all other loaders return 1.
- ```ColladaLoader``` still returns an object of objects instead of geometry/materials
- I skipped over the ```UTF8Loader``` because it looks like it is doing weird things with the ```onProgress``` event. I'll take a closer look at this one.

Other stuff:
- The ```PVRLoader``` seems like it was already broken ... Am I mistaken?
- The ```SceneLoader``` example has a broken shader in dev?