Version 1.1, cleaned

- moved the api configuration to different file
- split the playnext.js into two files:
--playnext.js doing the playlist control
--playnextbutton.js displaying the extension button on youtube
thumbnails
- rewrote the convertDuration function to  handle all possibilities in
ISO8601 duration standard, though I hope there are no over-one-year-long
videos on youtube ;-)
- wrote the comments
- general code cleaning