mozIJSSubScriptLoader Interface

 This method should only be called from JS!
 In JS, the signature looks like:
 rv loadSubScript (url [, obj]);
 @param url the url if the sub-script, it MUST be either a file:,
            resource:, or chrome: url, and MUST be local.
 @param obj an optional object to evaluate the script onto, it
            defaults to the global object of the caller.
 @retval rv the value returned by the sub-script