Updating Markdown class + use spl_autoload_register

spl_autoload_register now calls the GNUsocial_class_autoload function
instead of us replacing the magic __autoload($cls). This means we can
queue up other autoload functions, such as the one now used for extlib
functions which exist directly in the 'extlib/' folder or have proper
namespacing (which our new Markdown class does).

extlib/Michelf/MarkdownInterface.php

@@ -0,0 +1,37 @@
+<?php
+#
+# Markdown  -  A text-to-HTML conversion tool for web writers
+#
+# PHP Markdown
+# Copyright (c) 2004-2013 Michel Fortin
+# <http://michelf.com/projects/php-markdown/>
+#
+# Original Markdown
+# Copyright (c) 2004-2006 John Gruber
+# <http://daringfireball.net/projects/markdown/>
+#
+namespace Michelf;
+
+
+#
+# Markdown Parser Interface
+#
+
+interface MarkdownInterface {
+
+  #
+  # Initialize the parser and return the result of its transform method.
+  # This will work fine for derived classes too.
+  #
+  public static function defaultTransform($text);
+
+  #
+  # Main function. Performs some preprocessing on the input text
+  # and pass it through the document gamut.
+  #
+  public function transform($text);
+
+}
+
+
+?>