add istanbul test coverage tool + move code in src folder

Author: yao-dev

.babelrc

@@ -1,3 +1,8 @@
 {
-  "presets": ["env"]
+  "presets": ["env"],
+  "env": {
+    "test": {
+      "plugins": ["istanbul"]
+    }
+  }
 }

.gitignore

@@ -1,3 +1,6 @@
 node_modules
 .git
 .DS_Store
+npm-debug.log
+.nyc_output
+coverage

package.json

@@ -4,7 +4,8 @@
   "description": "Web development training with Coding Dojo : Katas + TDD (software craftmanship)",
   "main": "index.js",
   "scripts": {
-    "test": "jest --watch"
+    "test": "jest --watch",
+    "test:coverage": "NODE_ENV=test nyc --reporter=lcov --reporter=text jest src"
   },
   "repository": {
     "type": "git",
@@ -19,6 +20,8 @@
   "homepage": "https://github.com/yao-dev/katas#readme",
   "devDependencies": {
     "babel-core": "^6.26.0",
+    "babel-plugin-istanbul": "^4.1.5",
+    "nyc": "^11.4.1",
     "babel-preset-env": "^1.6.1",
     "jest": "^21.2.1"
   }