made simple-node-logger optional => moved to dev dependencies

TODO

@@ -1,4 +1,2 @@
 - emit an event whenever the rate limiter allows to run more jobs
 - more tests on concurrency
-- linter (eslint --fix ou prettier)
-- move simplelogger to dev dependencies

package.json

@@ -17,11 +17,9 @@
   },
   "author": "Adrien Joly",
   "license": "ISC",
-  "dependencies": {
-    "simple-node-logger": "^0.93.30"
-  },
   "devDependencies": {
     "jsdoc": "^3.5.5",
-    "mocha": "^4.0.1"
+    "mocha": "^4.0.1",
+    "simple-node-logger": "^0.93.30"
   }
 }

qyu.js

@@ -5,13 +5,12 @@
 // see https://github.com/nodejs/node/issues/9523#issuecomment-259303079
 
 const EventEmitter = require('events');
-const SimpleNodeLogger = require('simple-node-logger');
 const RateLimiter = require('./RateLimiter');
 
 const LOWEST_PRIO = 10;
 
 const DEFAULT_QUEUE_OPTIONS = {
-  log: SimpleNodeLogger.createSimpleLogger(),
+  log: { trace: () => {}, debug: () => {} }, // can be replaced by instance of simple-node-logger
   rateLimit: null,        // falsy => process in series. otherwise: max number of jobs to run within 1 second
   statsInterval: 1000,    // emit `stats` every second
 };