Substitute recursive variant tag within variadic templates (e.g. tuple)

Unspecialize variadic substitute on Arity to prevent ambiguity with boost::variant<...> partial specialization in recursive_variant.hpp; add test.

Add check for BOOST_NO_CXX11_HDR_TUPLE, in case we end up with a compiler with variadic templates but no std::tuple
Merge pull request #3 from ecatmur/make-recursive-variant-variadic

Substitute recursive variant tag within variadic templates (e.g. tuple)