Simple method to add a notice to the WP Admin

composer.json

{
  "name": "nathanielks/wp-admin-notice",
  "type": "library",
  "license": "GPL-2.0+",
  "description": "A simple class for displaying admin notices in the WordPress admin.",
  "homepage": "https://gist.github.com/nathanielks/8910601.git",
  "authors": [
    {
      "name": "Nathaniel Schweinberg",
      "email": "nathaniel@fightthecurrent.org",
      "homepage": "http://fightthecurrent.org"
    }
  ],
  "autoload": {
    "psr-4": { "": "" }
  }
}

WP_Admin_Notice.php

<?php
class WP_Admin_Notice {

    public $class = '';
    public $message = '';

    function __construct( $message, $class = 'updated' ){
        $this->class = $class;
        $this->message = $message;
        add_action( 'admin_notices', array( $this, 'output' ) );
        add_action( 'network_admin_notices', array( $this, 'output' ) );
    }

    function output(){
        echo '<div id="message" class="' . $this->class .'"><p>' . $this->message . '</p></div>';
    }
}