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.

There is a bug with template alias in MSVC. Firstly they are not as equivalent for overload resolution, this causes failure. Second they internal compiler error if you use them too complex. Once you have approved this patch I will send both copies to Microsoft for bug reporting. They almost certainly will not fix VS2013 though. I may need that to work, which is why I spent a full day trial and error to figure out this fix. It was frustrating.