Remove a couple of bugs, clarify a couple of minor points; from Kevlin

svn path=/trunk/boost/libs/conversion/; revision=14148
Handle unsigned __int64 for MSVC6

svn path=/trunk/boost/boost/cast.hpp; revision=14408
tru64cxx6.5 fixes

svn path=/trunk/boost/boost/cast.hpp; revision=14421
Patch from Gennadiy Rozental

svn path=/trunk/boost/boost/cast.hpp; revision=14621