More flexible functor copy subscription for EventSet

Yes, this allows the cool C++11 lambda usage :-)

I will likely not be doing these changes for FunctorReferenceBinder because
if you use that you already have enough boilerplate to afford having
exactly the right signature.
Add info on how to run tests on Windows
Fix cegui folder reference
CEGUI::OpenGL3Renderer::~OpenGL3Renderer changed from private to protected

This fixes build in python bindings issues with clang.
CMakeLists.txt modified to support add_directory()