[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/event/composer.json

@@ -0,0 +1,47 @@
+{
+    "name": "sabre/event",
+    "description": "sabre/event is a library for lightweight event-based programming",
+    "keywords": [
+        "Events",
+        "EventEmitter",
+        "Promise",
+        "Hooks",
+        "Plugin",
+        "Signal",
+        "Async"
+    ],
+    "homepage": "http://sabre.io/event/",
+    "license": "BSD-3-Clause",
+    "require": {
+        "php": ">=5.5"
+    },
+    "authors": [
+        {
+            "name": "Evert Pot",
+            "email": "me@evertpot.com",
+            "homepage": "http://evertpot.com/",
+            "role": "Developer"
+        }
+    ],
+    "support": {
+        "forum": "https://groups.google.com/group/sabredav-discuss",
+        "source": "https://github.com/fruux/sabre-event"
+    },
+    "autoload": {
+        "psr-4": {
+            "Sabre\\Event\\": "lib/"
+        },
+        "files" : [
+            "lib/coroutine.php",
+            "lib/Loop/functions.php",
+            "lib/Promise/functions.php"
+        ]
+    },
+    "require-dev": {
+        "sabre/cs": "~0.0.4",
+        "phpunit/phpunit" : "*"
+    },
+    "config" : {
+        "bin-dir" : "bin/"
+    }
+}