Removed MaterialResourceLoader, now use MaterialResourceHandler everywhere.
Main differences:
Material assets are now loaded like normal "file" assets via the asset registry. MaterialRequests use URL in format "asset://resource_id".
Asset.resource field will now be valid on assets. This will fix #14 when deployed.
Add nicer error messages if model tries to load material asset which isn't present.
Merge pull request #17 from playcanvas/feature/material-asset-resources

Removed MaterialResourceLoader replaced with MaterialResourceHandler