Fix keycode for space bar.
Remove link to non-existent stylesheet.
Wait for player before putting ball in motion.
Reset the ball speed each round.
Simplify index page.
Only preventDefault for keypressed used in game.

Preventing the default action for all keypresses conflicts with
some useful shortcuts in browsers.
Avoid resetting the ball once the game is over.

Also check if there are no more lives before decrementing the
lives counter. This prevents the number of lives being displayed
as -1 once the game has ended.
Center the "game over" text.