Initial implementation of HostEpoch

When host time is set, the diff between the BmcTime and the value is
saved to persistent storage;
When host time is retrieved, return the BmcTime plus the diff as
host's time.

Add the unit test cases for HostEpoch.

Change-Id: Ia55b93bfcba4f226ceaed8491136ea7afda7bd77
Signed-off-by: Lei YU <mine260309@gmail.com>
diff --git a/Makefile.am b/Makefile.am
index 3eef41a..0af260f 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -6,7 +6,8 @@
 
 libtimemanager_la_SOURCES = \
 	epoch_base.cpp \
-	bmc_epoch.cpp
+	bmc_epoch.cpp \
+	host_epoch.cpp
 
 phosphor_timemanager_SOURCES = \
 	main.cpp