step 4 - recycle
ok - add the bullets counter
ok - reduce npc life to zero shooting
ok - show dead sprite
- recyle to energy
- recycle to bullets
- test yellow bullet to reduce speed

step 5 - levels please
- define levels map + npc positions
- code boss behavior for levels
- count the time used to finish each level

step 6 - polish
- add help screen
- add backgrund image to the levels
- mute music