V0.2 JAM

Today's update:

• Music! (I try to do somthing not too borring but clairly I'm not musician...)
• Add difficulty system (speed of enemy increase, gap betwin spowning deacrease, flying enemies try to trigger you)
• Explosions now follow the scrolling (It's hurt less my understaning of physics this way ;) )

Next update should be essencially cosmetic.