added new sprite

added blue_explosion for future use with projectile trail effects.
pushed new asset to assetloader

asset: blue_explosion
rewrote the getNearestEnemyTank helper

- renamed it to getNearestTank
- now works properly on AI (e.g. homing missile now tracks the player
and other tanks)
updated the homing missile and chain powerups to use the rewritten getNearestTank aux