presence: Add JSON config class

Add a JSON config class that will be used to parse a given JSON config
file and generate the fan presence policies from that config.

Tested:
    N/A

Change-Id: Iac1703ea8d9e1e1d0d40e15a58145181d4f1a0f9
Signed-off-by: Matthew Barth <msbarth@us.ibm.com>
diff --git a/presence/Makefile.am b/presence/Makefile.am
index 28412ee..6145699 100644
--- a/presence/Makefile.am
+++ b/presence/Makefile.am
@@ -11,7 +11,8 @@
 	gpio.cpp \
 	psensor.cpp \
 	tach.cpp \
-	tach_detect.cpp
+	tach_detect.cpp \
+	json_config.cpp
 
 phosphor_fan_presence_tach_LDADD = \
 	$(SDBUSPLUS_LIBS) \