Replace existing autoloader with Composer-supplied

- Move Friendica namespace to `src`
- Move required `ezyang/htmlpurifier` to vendor
- Remove existing static autoloader
- Change boot.php reference to the autoloader

vendor/ezyang/htmlpurifier/library/HTMLPurifier/Node/Comment.php

@@ -0,0 +1,36 @@
+<?php
+
+/**
+ * Concrete comment node class.
+ */
+class HTMLPurifier_Node_Comment extends HTMLPurifier_Node
+{
+    /**
+     * Character data within comment.
+     * @type string
+     */
+    public $data;
+
+    /**
+     * @type bool
+     */
+    public $is_whitespace = true;
+
+    /**
+     * Transparent constructor.
+     *
+     * @param string $data String comment data.
+     * @param int $line
+     * @param int $col
+     */
+    public function __construct($data, $line = null, $col = null)
+    {
+        $this->data = $data;
+        $this->line = $line;
+        $this->col = $col;
+    }
+
+    public function toTokenPair() {
+        return array(new HTMLPurifier_Token_Comment($this->data, $this->line, $this->col), null);
+    }
+}