[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/vobject/lib/Component/VTimeZone.php

@@ -53,8 +53,7 @@ class VTimeZone extends VObject\Component {
             'LAST-MODIFIED' => '?',
             'TZURL'         => '?',
 
-            // At least 1 STANDARD or DAYLIGHT must appear, or more. But both
-            // cannot appear in the same VTIMEZONE.
+            // At least 1 STANDARD or DAYLIGHT must appear.
             //
             // The validator is not specific yet to pick this up, so these
             // rules are too loose.