Merge pull request #5 from ericniebler/develop

mpl::for_each works on nvidia gpu's

Nice, thanks.
Add constexpr to conversion operator for integral constants
Add constexpr to conversion operator for bool integral constants
Add test for constexpr conversion operator
Remove dependency on Detail by using Predef directly.
eliminate unnecessary workaround for modern msvc versions
Merge pull request #8 from ericniebler/has-xxx-msvc

eliminate unnecessary workaround for modern msvc versions
add back the has_xxx workaround for msvc if used with nvcc