Change the way materials are loaded in preparation for the new animation system.

This is work towards resolving issue #46.
Parse <channel> tags in the material loader.

Technically, custom material channels are now fully supported and issue #46 is resolved, however we still need to remove the old system and move the default renderer over to the new system. This will be happening in the next commits.
Have the default renderer use the new material system.
Move everything over to the new material system.