Update pong.kv

PongBalls' id is not needed at this point of the tutorial; moreover it had a confusing value that was different from the values later on in the text ('pong_ball')
Merge pull request #1151 from rogererens/patch-4

Update pong.kv
Merge branch 'master' of https://github.com/kivy/kivy
allow executing garden on windows.
Merge pull request #1167 from matham/garden_bat

Execute Garden on windows.