Update resource loading implementation to handle only one URL per name

This follows up on the previous API change that allowed only one URL per name
during script/style loading and transitions the underlying implementation to
match the API.

Progress on #25.
No, because if `spf.net.script.url_[name]` is `undefined`, then the value of `name` is used, and `name` can't be `undefined` because of the preceding `if (name)` line.