[boost] Boost::Python patch for decorated class method

I found that I could not compile any code that tried to export a class method to Python if the method was "decorated" with a calling convention like __cdecl (MSVC8 style).  Standalone functions work ok because the compiler matches them against RT(*) while class methods were trying to match against RT(ClassT::*).   I'm sure there's a concise way to make signature.hpp work with decorative keywords generally, so I am *not* proposing you accept my patch.  I know virtually nothing about template metaprogramming; my code is neither concise nor general.  However, the patch does demonstrate the problem and solution.   Best, -Richard