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.
These pretty much only hit the happy path cases. t'd be nice to see some tests for name mismatches (existing url and new name or vice versa) and multiple urls that are already partially loaded.