- Colladawriter does now reset correctly (calling writeMesh twice failed before and writeScene twice didn't write light-nodes second time)
- Add some callbacks that allow users to use custom-names on writing
(I'll probably add an example as well soon for how to use all that - the problem is that there don't seem to be 2 tools out there interpreting Collada the same way and so we have to be rather flexible on export. And yeah - our own import also still  has way to go...)

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@4268 dfc29bdd-3216-0410-991c-e03cc46cb475