Add Jest as a development dependency for unit tests

package.json

@@ -5,7 +5,7 @@
   "main": "main.js",
   "scripts": {
     "start": "node main",
-    "test": "echo \"Error: no test specified\" && exit 1"
+    "test": "jest"
   },
   "repository": {
     "type": "git",
@@ -44,6 +44,7 @@
     "x256": "0.0.2"
   },
   "devDependencies": {
-    "eslint": "^5.7.0"
+    "eslint": "^5.7.0",
+    "jest": "^23.6.0"
   }
 }