[boost] Yet another shot at reflection/introspection

I have committed a new library to the sandbox. It's called "introspection". It's another shot at implementing reflection. I am aware that similar work already exists, like Jeremy Pack's reflection library and Boost.Mirror (although this one currently supports only compile-time reflection). It's a work in progress - the testsuite is the doc at this point ;-) - and maybe it will never be completed/submitted. My primary goal is not to implement reflection per se, I'd be happy to leave the honor to Jeremy but currently it's not clear that his work can serve as a basis for my future developments (multimethods, generic object manipulation). Jean-Louis