Add unit tests for StringValue and Value.

Add unit test framework to CMakeLists.txt.
Fix a couple of bugs found by unit tests.

Change-Id: I2092a7f0570fae0f19f9e083c4837ccefcc4ca1a

test/unit/main.cpp

@@ -0,0 +1,8 @@
+#include "gtest/gtest.h"
+
+int main(int argc, char **argv)
+{
+    ::testing::InitGoogleTest(&argc, argv);
+    int ret = RUN_ALL_TESTS();
+    return ret;
+}