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_namespaces.php

@@ -6,4 +6,6 @@ $vendorDir = dirname(dirname(__FILE__));
 $baseDir = dirname($vendorDir);
 
 return array(
+    'HTMLPurifier' => array($vendorDir . '/ezyang/htmlpurifier/library'),
+    'Detection' => array($vendorDir . '/mobiledetect/mobiledetectlib/namespaced'),
 );