Db.prototype.addUser = function(username, password, callback) {
  // Use node md5 generator
  var md5 = crypto.createHash('md5');
  // Generate keys used for authentication
  md5.update(username + ":mongo:" + password);
  var userPassword = md5.digest('hex');
  // Fetch a user collection
  this.collection(DbCommand.SYSTEM_USER_COLLECTION, function(err, collection) {
    // Insert the user into the system users collections
    collection.insert({user: username, pwd: userPassword}, {safe:true}, function(err, documents) {
      callback(err, documents);
    });
  });
};