add a new range adaptor boost::adaptors::ref_unwrapped

The new adaptor is like boost::adaptors::indirected, but for
std::reference_wrapper values (instead of pointer). It calls
.get() on every value and returns the result. It is useful for
range-based iteration of ranges of std::reference_wrapper (or
related) types.

test/adaptor_test/ref_unwrapped_example.cpp

@@ -0,0 +1,47 @@
+// Boost.Range library
+//
+//  Copyright Robin Eckert 2015. Use, modification and distribution is
+//  subject to the Boost Software License, Version 1.0. (See
+//  accompanying file LICENSE_1_0.txt or copy at
+//  http://www.boost.org/LICENSE_1_0.txt)
+//
+//
+// For more information, see http://www.boost.org/libs/range/
+//
+//[ref_unwrapped_example
+#include <boost/range/adaptor/ref_unwrapped.hpp>
+#include <iostream>
+#include <vector>
+
+struct example
+{
+  int value;
+};
+
+int main(int argc, const char* argv[])
+{
+//<-
+#if !defined(BOOST_NO_CXX11_DECLTYPE) \
+ && !defined(BOOST_NO_CXX11_RANGE_BASED_FOR) \
+ && !defined(BOOST_NO_CXX11_UNIFIED_INITIALIZATION_SYNTAX) \
+ && !defined(BOOST_NO_CXX11_AUTO_DECLARATIONS)
+//->
+    using boost::adaptors::ref_unwrapped;
+
+    example one{1};
+    example two{2};
+    example three{3};
+
+    std::vector<std::reference_wrapper<example> > input{one, two, three};
+
+    for (auto&& entry : input | ref_unwrapped)
+    {
+      std::cout << entry.value;
+    }
+
+    return 0;
+//<-
+#endif
+//->
+}
+//]