Sounds, Music Track and fixes.

Added Sounds Effects for: Cannon Fire, Explosion, Splash.
Added looping music track from luigisounds.newgrounds.com
Ships now spawn 200px away from the edges of the screen, rather than a
set distance from the centre.
Fixed problem with the Ship spawning interval always being 5 seconds.
Fixed problem with the Ships and Row Boats rotation (instead of using
the StepTo util function, they are now using RotateTowards).
Moved AI under Entities rather than Entities/Characters.
Fixed missing graphics from How To Play.