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.

Yes I did. Any use of template alias for rebind causes the internal compiler error. I also tried a shim type around rebind with special constructor overloads to make it acceptable for implicit construction. These caused some sfinae boost result of to fail. I tried modifying some of these but the work became large, so I gave up and simply modified rebind to use the old fashioned pattern. That caused the least change.