Cache wrapping in background script
    
    A part of issue #40 handling.
    
    Background script caches the wrapping to be applied and also the
    information about CSP presence (#25). The hope is to improve speed and
    consequently, apply wrapping before page scripts kick in. Nevertheless,
    there the messages between background and page scripts is still
    asynchronous. Hence, the possibility to use or store unwrapped objects
    by page scripts early reamins. See #40 for more details.