Add a LeakHunter class which can be enabled with compile-flag _IRR_COMPILE_WITH_LEAK_HUNTER_ to find leaking IReferenceCounted objects.
Will break OSX compiling for now as that project file is not yet updated. I hope other project files are all fixed correctly.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@4425 dfc29bdd-3216-0410-991c-e03cc46cb475