add: unit and integration testing

app/node_modules/fast-json-stable-stringify/example/value_cmp.js

@@ -0,0 +1,7 @@
+var stringify = require('../');
+
+var obj = { d: 6, c: 5, b: [{z:3,y:2,x:1},9], a: 10 };
+var s = stringify(obj, function (a, b) {
+    return a.value < b.value ? 1 : -1;
+});
+console.log(s);