Move class about Sprite to renderer.sprite.
Merge branch 'master' of https://github.com/cchampet/Java_nanowars.git

Conflicts:
	src/renderer/UnitSprite.java
Add ProjectileSprite.
(use "proj_damage.png" image for all projectiles)
Merge branch 'master' of https://github.com/cchampet/Java_nanowars.git

Conflicts:
	src/engine/tower/Tower.java
	src/renderer/TowerSprite.java