Add sea mine art
Add directions for up and down
Remove unused method on Entity class
Optimize code
Create rocket explosion effect
Use entity position on bullet.
Add new direction.
Rocket bullet remove implements IExplosion.
On hit create rocket explosion.
Fix rocket gun offset after bullet use entity position.
Const fields
Add sea mine and sea mine spawner
Clear code
Add sea mine spawner on level two.
Set background image static final.
Merge pull request #18 from KBaluh/develop

Sea mine and RocketExplosion for bullet