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/composer/autoload_files.php

@@ -0,0 +1,10 @@
+<?php
+
+// autoload_files.php @generated by Composer
+
+$vendorDir = dirname(dirname(__FILE__));
+$baseDir = dirname($vendorDir);
+
+return array(
+    '2cffec82183ee1cea088009cef9a6fc3' => $vendorDir . '/ezyang/htmlpurifier/library/HTMLPurifier.composer.php',
+);