added isClear method to Sprite, refactored
added isClear method to Sprite, refactored

Couchfriends/HTML5-Asteroids,dmcinnes/HTML5-Asteroids,BabluKumar17/HTML5-Asteroids,nevdev/HTML5-Asteroids,WildDogTeam/demo-js-starwars,Japskua/HTML5-Asteroids,Legioneer/HTML5-Asteroids,nevdev/HTML5-Asteroids,trnl/astrds,Legioneer/HTML5-Asteroids,Couchfriends/HTML5-Asteroids,honu12/HTML5-Asteroids,googlearchive/HTML5-Asteroids,googlearchive/HTML5-Asteroids,mimming/mmo-asteroids,thatandromeda/HTML5-Asteroids,bala92/HTML5-Asteroids,mimming/mmo-asteroids,quarkgluons/Asteroids,Japskua/HTML5-Asteroids,dmcinnes/HTML5-Asteroids,thatandromeda/HTML5-Asteroids,WildDogTeam/demo-js-starwars,bala92/HTML5-Asteroids,quarkgluons/Asteroids,cindyaddoil/HTML5-Asteroids,trnl/astrds,mimming/mmo-asteroids,honu12/HTML5-Asteroids,cindyaddoil/HTML5-Asteroids,BabluKumar17/HTML5-Asteroids