Convert COLLADA loader into COLLADA class

[cjcliffe]

Now that the COLLADA loader is matured a bit it's time to start dividing it up further into a class that can initialize itself from a COLLADA file and then be used as more of a resource than just a one-shot scene.

Right now we parse and load all the scenes from a COLLADA file but just return the default one and can't extract individual resources without loading the whole thing; this won't be sufficient moving forward.