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/wscript b/wscript
index 71754fe..0197ae8 100644
--- a/wscript
+++ b/wscript
@@ -169,6 +169,19 @@
         "utils/ndn-limits.h",
         "utils/ndn-rtt-estimator.h",
 
+        # "utils/tracers/ipv4-app-tracer.h",
+        # "utils/tracers/ipv4-l3-tracer.h",
+        # "utils/tracers/ipv4-rate-l3-tracer.h",
+        # "utils/tracers/ipv4-seqs-app-tracer.h",
+
+        "utils/tracers/l2-rate-tracer.h",
+        "utils/tracers/l2-tracer.h",
+        "utils/tracers/ndn-app-delay-tracer.h",
+        "utils/tracers/ndn-cs-tracer.h",
+        "utils/tracers/ndn-l3-aggregate-tracer.h",
+        "utils/tracers/ndn-l3-tracer.h",
+        "utils/tracers/ndn-l3-rate-tracer.h",
+
         "apps/callback-based-app.h",
         ]