[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/Version.php

@@ -14,6 +14,6 @@ class Version {
     /**
      * Full version number
      */
-    const VERSION = '1.4.1';
+    const VERSION = '1.5.0';
 
 }