title: boost::asio with boost::unique_future
body: <p>According to <a href="http://www.boost.org/doc/libs/1_55_0/doc/html/boost_asio/overview/cpp2011/futures.html" rel="noreferrer">http://www.boost.org/doc/libs/1_55_0/doc/html/boost_asio/overview/cpp2011/futures.html</a>, we can use boost::asio with <code>std::future</code>. But I couldn't find any information about working with <code>boost::unique_future</code>, which has more functions, such as <code>then()</code>. How can I use?</p>