VS14 CTP 3 support

Note this depends on the patch to Boost adding VS14 config support at https://svn.boost.org/trac/boost/ticket/10489.

It's actually very close to working on VS2013 too (see https://ci.nedprod.com/view/Boost%20Thread-Expected-Permit/job/Boost.Expected%20Test%20Win8%20VS2013/4/consoleFull). The only obstacle is the lack of unrestricted unions support, apart from that it appears to compile without issue now. Any suggestions on how to bypass the need for unrestricted unions would be welcome.

Please, could you use conditional compilation here.