Make HTTPS Everywhere compatible with e10s (multiprocess firefox)

Work in progress. As of now, rewrites work and the basic UI features work.

fixes #447 #510 

TODO:
* check for backwards compatibility
I'm not super familiar with these APIs, so catching up by reading docs:

https://developer.mozilla.org/en-US/Add-ons/Code_snippets/Tabbed_browser
> You can access gBrowser only after the browser window is fully loaded. If you need to do something with gBrowser right after the window is opened, listen for the load event and use gBrowser in the event listener.

This seems to imply that the reference to gBrowser here is incorrect? Maybe need to check aFlags and aWebProgress before gBrowser.selectedBrowser?