Fixed a bug and brought some changes

Hey,
The newest version was not working for me and some other people.
I fixed it, also by changing xpath selectors (which I never used) to DOM traditionnal selectors.
I took the chance to:
  - refactor a little the button generation
  - set the buttons under the video
  - make the button download immediately, instead of opening the video in another tab
  - add some credits
Hope you like it!