Add eslint config and fix linting issues

src/util/Util.js

@@ -7,29 +7,29 @@ const has = (o, k) => Object.prototype.hasOwnProperty.call(o, k);
  */
 class Util {
 
-  constructor() {
-    throw new Error(`The ${this.constructor.name} class may not be instantiated.`);
-  }
-
-  /**
-  * Sets default properties on an object that aren't already specified.
-  * @param {Object} def Default properties
-  * @param {Object} given Object to assign defaults to
-  * @returns {Object}
-  * @private
-  */
-  static mergeDefault(def, given) {
-    if (!given) return def;
-    for (const key in def) {
-      if (!has(given, key) || given[key] === undefined) {
-        given[key] = def[key];
-      } else if (given[key] === Object(given[key])) {
-        given[key] = Util.mergeDefault(def[key], given[key]);
-      }
+    constructor() {
+        throw new Error(`The ${this.constructor.name} class may not be instantiated.`);
     }
 
-    return given;
-  }
+    /**
+     * Sets default properties on an object that aren't already specified.
+     * @param {Object} def Default properties
+     * @param {Object} given Object to assign defaults to
+     * @returns {Object}
+     * @private
+     */
+    static mergeDefault(def, given) {
+        if (!given) return def;
+        for (const key in def) {
+            if (!has(given, key) || given[key] === undefined) {
+                given[key] = def[key];
+            } else if (given[key] === Object(given[key])) {
+                given[key] = Util.mergeDefault(def[key], given[key]);
+            }
+        }
+
+        return given;
+    }
 }
 
 module.exports = Util;