Added VS14 build file.
Replaced all use of constexpr with BOOST_CONSTEXPR
Apart from many ICEs in VS14 CTP3, now compiles cleanly :)
Worked around the first MSVC ICE in both VS2013 and VS14 CTP1, it doesn't like returning a template alias from a function.
Fixed the other MSVC ICE, but now have quite a few failing tests.