optimize asset reference filenames

Currently asset references get dumped with generic filenames like `fb32317_part_1.js`, `fb32317_part_2.js` etc.
Unfortunately these filenames don't tell you which asset reference they represent which would be very useful for debugging purposes IMO (especially when you have lots of assets).
So instead the asset reference name should be used instead of `part`: `fb32317_jquery_1.js`, `fb32317_bootstrap_2.js`.

What do you think?