Added the "Autopoke button" to Autopoke.user.js

- Added an "Autopoke button" to start/stop autopoking.
- Refactored the code
- Added a max-height and a scrollbar to the "Autopoke log".
Improved style of the log

- Copied the Facebook style using its classes.
- Added a fixed height with auto-appearing scrollbar.
- The Scrollbar is always at the bottom of the log.