User authentication implemented

Closes #5 (closed)

models/todo.js

+const mongoose = require('mongoose');
+
+const TodoSchema = new mongoose.Schema({
+  description: { type: String, required: true },
+  isDone: { type: Boolean, default: false },
+  userId: { type: mongoose.Schema.Types.ObjectId, ref: 'User', required: true }
+});
+
+module.exports = mongoose.model('Todo', TodoSchema);