Add introductions to wave of ennemies
Fix sizes for better collisions
Add weight to baddies and implement some waves
Batter starfield feeling
Baddies tuning
Fix Douo rocket problem
Update todo
Make acceleration a pattern that can be configured
Stats and gameplay change

Now the gameplay is a bit different : ennemies won't reappear after they
pass the bottom of the screen. This is the first towards executing the
shooter-defense gameplay.

Also added a stats module to store the different kind of informations
during a game, such as player deaths, ennemy kills and of course ennemy
wins.
Normalize it!