Merge branch 'v4.0.0-WIP' into develop

Conflicts:
	.gitignore
	.travis.yml
	README.md
	composer.json
	phpunit.xml
	sql/mysql.sql
	src/League/OAuth2/Server/Grant/RefreshToken.php
	src/League/OAuth2/Server/Resource.php
	src/League/OAuth2/Server/Storage/SessionInterface.php
	src/League/OAuth2/Server/Util/Request.php
	src/Util/KeyAlgorithm/DefaultAlgorithm.php
	tests/resource/ResourceServerTest.php
	tests/util/RedirectUriTest.php
	tests/util/RequestTest.php
	tests/util/SecureKeyTest.php

src/Util/KeyAlgorithm/DefaultAlgorithm.php

@@ -0,0 +1,46 @@
+<?php
+/**
+ * OAuth 2.0 Secure key generator
+ *
+ * @package     league/oauth2-server
+ * @author      Alex Bilbie <hello@alexbilbie.com>
+ * @copyright   Copyright (c) Alex Bilbie
+ * @license     http://mit-license.org/
+ * @link        https://github.com/thephpleague/oauth2-server
+ */
+
+namespace League\OAuth2\Server\Util\KeyAlgorithm;
+
+class DefaultAlgorithm implements KeyAlgorithmInterface
+{
+    protected static $algorithm;
+
+    /**
+     * {@inheritdoc}
+     */
+    public function generate($len = 40)
+    {
+        return self::getAlgorithm()->make($len);
+    }
+
+    /**
+     * @param KeyAlgorithmInterface $algorithm
+     */
+    public static function setAlgorithm(KeyAlgorithmInterface $algorithm)
+    {
+        self::$algorithm = $algorithm;
+    }
+
+    /**
+     * @return KeyAlgorithmInterface
+     */
+    public static function getAlgorithm()
+    {
+        if (!self::$algorithm) {
+
+            self::$algorithm = new DefaultAlgorithm();
+        }
+
+        return self::$algorithm;
+    }
+}