Revert "Drop custom autoload"

This reverts commit aca3d1c.

Turns out this file is used by DomPdf for his manual autoloading
mechanism.

src/autoload.php

@@ -0,0 +1,17 @@
+<?php
+/**
+ * @package php-svg-lib
+ * @link    http://github.com/PhenX/php-svg-lib
+ * @author  Fabien Ménager <fabien.menager@gmail.com>
+ * @license GNU LGPLv3+ http://www.gnu.org/copyleft/lesser.html
+ */
+
+spl_autoload_register(function($class) {
+  if (0 === strpos($class, "Svg")) {
+    $file = str_replace('\\', DIRECTORY_SEPARATOR, $class);
+    $file = realpath(__DIR__ . DIRECTORY_SEPARATOR . $file . '.php');
+    if (file_exists($file)) {
+      include_once $file;
+    }
+  }
+});