Basic function

shoot, move, collide with enemies
Player explosion

1. Stop shoot music when player plane crashes
2. Add Explosion effect when player plane crashes
map size
new image source
big bug in drawing bullets

Bug: If the number of bullets greater than capacity of bullets, the
screen will be stuck
Fixed big bug in flying objects

Bug:
1. The bullets and enemies get stuck for 1 sec
2. If there are too many bullets, some of them will stay on the screen
for a while
Player explosion music
detect collide between bullets and enermies
Merge pull request #2 from yl-1993/master

Basic function