add tuning enable variable

Add a variable that when set, enables tuning logging.  This variable is
set to false.

Tuning can be enabled via "-t" "--tuning" on the command line.
With a parameter is the path where to write the logging information.

Change-Id: I6eb8035d56cc3301face21e9375c02fc9fcc5b31
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/Makefile.am b/Makefile.am
index 9a31124..3a1b52b 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -60,6 +60,7 @@
 	pid/zone.cpp \
 	pid/util.cpp \
 	pid/pidthread.cpp \
+	pid/tuning.cpp \
 	threads/busthread.cpp \
 	build/buildjson.cpp \
 	experiments/drive.cpp