Fix writeScene change in order to write the given node and its descendants, not only the list of children to the scene file.
Add support for loading a scene file as childs of a given node. This will ignore the scene attributes and load all nodes of the scene, storing them as children of a given parent node. The rest will stay as before, including the default behavior to load scene attributes and all nodes directly into the scene manager.
Test cases added for both methods loadScene and saveScene. In order to find a node in the example scene from tutorial 15 I have added a name and ID to one of the scene elements.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@3532 dfc29bdd-3216-0410-991c-e03cc46cb475