Refactor to unify spf.net.script and spf.net.style.

Moves common code from spf.net.script and spf.net.style into spf.net.resource.
Reduces duplication and enforces consistency between the script and style
execution/loading models.

Progress on #25.
On the other hand, these are now pretty trivial functions. I, for one, wouldn't mind removing their tests altogether.