Particles: Make attached particle spawners respect the parent's yaw

Position, velocity and acceleration vectors of particles are rotated
by the yaw of the parent object so that they are truly relative to it.
Clarify new attached particle spawner behavior in lua_api.txt.

src/clientobject.h

@@ -61,6 +61,7 @@ public:
 	virtual bool getCollisionBox(aabb3f *toset){return false;}
 	virtual bool collideWithObjects(){return false;}
 	virtual v3f getPosition(){return v3f(0,0,0);}
+	virtual float getYaw() const {return 0;}
 	virtual scene::ISceneNode *getSceneNode(){return NULL;}
 	virtual scene::IMeshSceneNode *getMeshSceneNode(){return NULL;}
 	virtual scene::IAnimatedMeshSceneNode *getAnimatedMeshSceneNode(){return NULL;}