Changed permissions and began background implement

- Changed "tab" permission for "activeTab".
- Removed "windows" permission.
- Changed "http(s)://*/*" permissions for "<all_urls>".
- Began to implement a background script to run the logic instead of the
popup.js.
Changed ":load:" keyword for "//:load:"

So the script can be copied and pasted as is, without having to remove
":load:", which is a syntax error for JS...
Moved executeScript & inject funcs to background.js