Support nodes with mirrored transforms (negative scales).

Thanks to scottnc27603 for fixing a bug for particle systems
Fix ColladaLoader to identify transparent textures as such
Fix ColladaLoader to assign specular maps to the specularMap property instead of overwriting the map property
Add support for bump maps to the ColladaLoader
Update collada loader to use the collada ID as the threejs ID to fix a bug in loading the animations
Experimental support in the collada loader for assigning textures to multiple sets of texture coordinates