Finished iterator_adaptor tutorial

Added example test code
Rolled forward old counting_iterator abstract for boost docs.


[SVN r21649]

example/Jamfile

@@ -9,4 +9,5 @@ test-suite iterator_examples
   : [ run reverse_iterator.cpp ]
     [ run node_iterator1.cpp ]
     [ run node_iterator2.cpp ]
+    [ run node_iterator3.cpp ]
       ;