Add a definition for the copy constructor of dynamic_bitset::reference

Reason: Implicit definition is deprecated, because the class has a
user-declared copy assignment operator.

This closes issue #62.

include/boost/dynamic_bitset/dynamic_bitset.hpp

@@ -187,6 +187,13 @@ public:
         void operator&();
 
     public:
+        //!     Copy constructor.
+        //!
+        //!     Constructs a `reference` which refers to the same bit as
+        //!     `other`.
+        // -------------------------------------------------------------------
+        reference( const reference & other );
+
         //!     See the class description.
         // -------------------------------------------------------------------
         operator bool() const;

include/boost/dynamic_bitset/impl/dynamic_bitset.tpp

@@ -24,6 +24,9 @@ dynamic_bitset< Block, Allocator >::reference::reference( block_type & b, int po
 {
 }
 
+template< typename Block, typename Allocator >
+dynamic_bitset< Block, Allocator >::reference::reference( const reference & other ) = default;
+
 template< typename Block, typename Allocator >
 dynamic_bitset< Block, Allocator >::reference::
 operator bool() const