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
Dummy commit to master to trigger gh-pages rebuild.
Merge branch 'master' of https://github.com/TerraPass/2048
Delete a.txt
Merge pull request #1 from TerraPass/master