Add support for VC++ 14

All the config tests pass, with the exception of the user-defined literal test, which passes only if "static constexpr" is substituted with "static BOOST_CONSTEXPR_OR_CONST" and "constexpr" is substituted with "BOOST_CONSTEXPR".

See
http://blogs.msdn.com/b/vcblog/archive/2014/06/03/visual-studio-14-ctp.aspx
and
http://blogs.msdn.com/b/somasegar/archive/2014/05/28/first-preview-of-visual-studio-quot-14-quot-available-now.aspx