More comprehensive solution to issue of iframes...

 ...not getting content scripts re-injected

Fixes #16 (again).
I think there is some redundant logic here?

- If a frame completes loading before the main frame it gets put in framesLoaded.
- framesLoaded is used to skip injecting into frames that load later.
- but it's impossible for the same frame to get onCompleted more than once.

am I misreading?