Update README.md, centered screenshot!

I think screenshot is better when it is centered.
Merge pull request #85 from MaciejCzyzewski/patch-2

Update README.md, centered screenshot!
Update README.md
Add Restart button below game grid
Replace best score with best tile reached
Update winning logic to go beyond 2048

- start showing message at 512
- improve message to show what tile value was reached
Update README with details about 2048+
Merge branch '2048+' into gh-pages