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
change game to +1 / -1

* change gameplay to generate tiles with +1 or -1
* game is won when score goes to zero
keep track of balanced score and use it to drive randomness a bit
renderNumber function
phantom grid to support tiles that move and then fade out
start with 3 tiles
better icons
updated text
Merge branch 'refs/heads/master' into gh-pages

Conflicts:
	js/html_actuator.js