Either fix internet_off or add a new decorator to disable internet #17

Currently the internet_off decorator indicates that a test will only run if --remote-data is not provided or --remote-data=none. It would be more useful to have a decorator that actually disables network access for the scope of that test.

To accomplish this we could either update internet_off, or we could add a new decorator (maybe disable_internet). As far as I know, no one is using internet_off yet, so we probably don't have to worry about API breakage.