BGE : addObject in python without reference object.

Making the reference argument optional for the addObject function.
```
scene.addObject("Cube")
```

This allows to keep the rotation, scale and position of the original object.
To avoid layer problems with lights if the reference arguments is None, the new object have the same layer than the active layers in scene.

Reviewers: lordloki, moguri, hg1, sybren

Reviewed By: hg1, sybren

Subscribers: agoose77

Projects: #game_engine

Differential Revision: https://developer.blender.org/D1222

doc/python_api/rst/bge_types/bge.types.KX_Scene.rst

@@ -143,9 +143,9 @@ base class --- :class:`PyObjectPlus`
 
       :arg object: The (name of the) object to add.
       :type object: :class:`KX_GameObject` or string
-      :arg reference: The (name of the) object which position, orientation, and scale to copy.
+      :arg reference: The (name of the) object which position, orientation, and scale to copy (optional), if the object to add is a light and there is not reference the light's layer will be the same that the active layer in the blender scene.
       :type reference: :class:`KX_GameObject` or string
-      :arg time: The lifetime of the added object, in frames. A time of 0 means the object will last forever.
+      :arg time: The lifetime of the added object, in frames. A time of 0 means the object will last forever (optional).
       :type time: integer
       :return: The newly added object.
       :rtype: :class:`KX_GameObject`