shared_ptr -> std::shared_ptr

This is one of the first commits in a plan to remove all `using namespace std;` lines in the entire codebase as it is considered anti-pattern today.

Minecraft.World/Creeper.h

@@ -45,7 +45,7 @@ protected:
 
 public:
 	virtual void die(DamageSource *source);
-	virtual bool doHurtTarget(shared_ptr<Entity> target);
+	virtual bool doHurtTarget(std::shared_ptr<Entity> target);
 	virtual bool isPowered();
     float getSwelling(float a);