refactor(injection): replace background.js and content_script.js with */inject.js

- remove background.js and content_script.js

- add builds/chromium/inject.js, it only injects sources, nothing more
- add firefox/chrome/content/inject.js, it only injects sources, nothing more
- add maxthon/inject.js, it does nothing :(
- add opera/includes/inject.js, it does nothing :(