Add optional serialization support

This implemenents non-intrusive serialization support to dynamic_bitset<> based on Boost Serialization

 - it supports archives that require named elements
 - it gets private member access using a nested friend class (to decouple the interface entirely)
 - relies on boost/serialization/vector.hpp for actual implementation

include/boost/dynamic_bitset/dynamic_bitset.hpp

@@ -348,6 +348,10 @@ public:
 
 #endif
 
+public:
+    // forward declaration for optional zero-copy serialization support
+    class serialize_impl;
+    friend class serialize_impl;
 
 private:
     BOOST_STATIC_CONSTANT(block_width_type, ulong_width = std::numeric_limits<unsigned long>::digits);

include/boost/dynamic_bitset/serialization.hpp

@@ -0,0 +1,46 @@
+// -----------------------------------------------------------
+//
+// Copyright (c) 2015 Seth Heeren
+//
+// Distributed under 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)
+//
+// -----------------------------------------------------------
+
+#ifndef BOOST_DYNAMIC_BITSET_SERIALIZATION_HPP
+#define BOOST_DYNAMIC_BITSET_SERIALIZATION_HPP
+
+#include "boost/dynamic_bitset/dynamic_bitset.hpp"
+#include <boost/serialization/vector.hpp>
+
+namespace boost {
+
+    // implementation for optional zero-copy serialization support
+    template <typename Block, typename Allocator>
+        class dynamic_bitset<Block, Allocator>::serialize_impl
+        {
+            public:
+                template <typename Ar> 
+                static void serialize(Ar& ar, dynamic_bitset<Block, Allocator>& bs, unsigned) {
+                    ar & serialization::make_nvp("m_num_bits", bs.m_num_bits)
+                       & serialization::make_nvp("m_bits", bs.m_bits);
+                }
+        };
+
+}
+
+// ADL hook to Boost Serialization library
+namespace boost {
+    namespace serialization {
+
+        template <typename Ar, typename Block, typename Allocator>
+            void serialize(Ar& ar, dynamic_bitset<Block, Allocator>& bs, unsigned version) {
+                dynamic_bitset<Block, Allocator>::serialize_impl::serialize(ar, bs, version);
+            }
+
+    } // namespace serialization
+} // namespace boost
+
+#endif // include guard
+