[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/Writer.php

@@ -127,7 +127,7 @@ class Writer extends XMLWriter {
 
             if (array_key_exists($namespace, $this->namespaceMap)) {
                 $result = $this->startElementNS(
-                    $this->namespaceMap[$namespace] === '' ?  null : $this->namespaceMap[$namespace],
+                    $this->namespaceMap[$namespace] === '' ? null : $this->namespaceMap[$namespace],
                     $localName,
                     null
                 );