Add Jest depdendency

package.json

@@ -8,7 +8,8 @@
     "url": "git+https://github.com/mymonero/mymonero-core-js.git"
   },
   "scripts": {
-    "format": "find . -name '*.js*' | xargs prettier --write --config ./.prettierrc --config-precedence file-override"
+    "format": "find . -name '*.js*' | xargs prettier --write --config ./.prettierrc --config-precedence file-override",
+    "test": "jest"
   },
   "keywords": [
     "monero",
@@ -31,5 +32,8 @@
   "homepage": "https://github.com/mymonero/mymonero-core-js#readme",
   "dependencies": {
     "async": "^2.6.0"
+  },
+  "devDependencies": {
+    "jest": "^23.1.0"
   }
 }