[FEATURE] Add config and use composer autoloader.

We use composer already to install SabreDAV. Include config
composer.(json|lock) to install and manage more dependencies
in future.
Also provide PSR-4 autoloading for our namespaced classes, e.g.
"Zotlabs\". To regenerate autoloader maps use:
$ composer install --optimize-autoloader --no-dev

We could also remove the whole vendor/ folder from our repository, but
that would need changes in deployment and how to install hubs and needs
more discussion first.

vendor/sabre/xml/lib/Element/XmlFragment.php

@@ -2,9 +2,9 @@
 
 namespace Sabre\Xml\Element;
 
+use Sabre\Xml\Element;
 use Sabre\Xml\Reader;
 use Sabre\Xml\Writer;
-use Sabre\Xml\Element;
 
 /**
  * The XmlFragment element allows you to extract a portion of your xml tree,