The Sprites class has been given direct access to the screen buffer so it
doesn't need a pointer to been given to it.
Also updated README.md and keywords.txt, and made minor indent changes in
the .h files.
- Version changed to 2.1.0
- Added sprite, button and collision functions from ArduboyExtra
- Added drawCompressed() function from Arglib
- Updated LICENSE.txt
- Removed CONTRIBUTORS.md (now covered by LICENSE.txt)