tracers+docs: Correcting compilation and installation of trace helpers

Also in this commit an example on how to use trace helpers, including
how to build graphs using R.
diff --git a/examples/wscript b/examples/wscript
index 5c478a7..67cf5ca 100644
--- a/examples/wscript
+++ b/examples/wscript
@@ -19,3 +19,6 @@
             'custom-strategies/custom-strategy.cc',
             'ndn-congestion-alt-topo-plugin.cc'
             ]
+
+        obj = bld.create_ns3_program('ndn-tree-tracers', ['ndnSIM'])
+        obj.source = 'ndn-tree-tracers.cc'