Allow paths relative to a resource in a subpath.

[supersaiyen]

if the contents is on a sub path, and the mapping is relative from there, like a CSS file including a font in a sub dir, we need to remove the subpath of the file being mapped, since it will not match the resource

[supersaiyen]

Longest pull request cycle ever... I've replaced all the string manipulation with the path module.

[jhuesos]

I think my PR #24 solves this problem using the path module. Could you check (if you still need this) to see if my PR solves your problem?