utils/tracers: Modifying (simplifying) interface for ndnSIM tracers

Now most of the tracers internally keep created instances in a global
variable.  From now on, to start tracing, one needs just install a
necessary tracer, like this:

    ndn::L3RateTracer::InstallAll ("rate-trace.txt", Seconds (0.5));

Because of the change, it became possible to use the same tracers inside
python scripts.

Also, this commits contains corresponding documentation updates.
diff --git a/utils/tracers/l2-rate-tracer.h b/utils/tracers/l2-rate-tracer.h
index de6ffcb..8dba680 100644
--- a/utils/tracers/l2-rate-tracer.h
+++ b/utils/tracers/l2-rate-tracer.h
@@ -58,7 +58,7 @@
    *          for the lifetime of simulation, otherwise SEGFAULTs are inevitable
    *
    */
-  static boost::tuple< boost::shared_ptr<std::ostream>, std::list<Ptr<L2RateTracer> > >
+  static void
   InstallAll (const std::string &file, Time averagingPeriod = Seconds (0.5));
 
   void