added powerup functionality for extra life
uses power_up image file and remove useless console.log statement
adjust powerup starting position to roughly at brick's centre bottom
ensures no more than 1 powerup at the same time
change powerupExists flag back to flase if player did not manage to catch the powerup
Major refactoring of dropping item for extensibility
Major refactoring of dropping item for extensibility
Merge branch 'power-up-extra-life' of github.com:nsue/multiplayer-breakout into power-up-extra-life

* 'power-up-extra-life' of github.com:nsue/multiplayer-breakout:
  Major refactoring of dropping item for extensibility
Set each drop chance to 5%
Merge pull request #56 from nsue/power-up-extra-life

Power up extra life