[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/http/composer.json

@@ -0,0 +1,43 @@
+{
+    "name": "sabre/http",
+    "description" : "The sabre/http library provides utilities for dealing with http requests and responses. ",
+    "keywords" : [ "HTTP" ],
+    "homepage" : "https://github.com/fruux/sabre-http",
+    "license" : "BSD-3-Clause",
+    "require" : {
+        "php"          : ">=5.4",
+        "ext-mbstring" : "*",
+        "sabre/event"  : ">=1.0.0,<4.0.0",
+        "sabre/uri"    : "~1.0"
+    },
+    "require-dev" : {
+        "phpunit/phpunit" : "~4.3",
+        "sabre/cs" : "~0.0.1"
+    },
+    "suggest" : {
+        "ext-curl" : " to make http requests with the Client class"
+    },
+    "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-http"
+    },
+    "autoload" : {
+        "files" : [
+            "lib/functions.php"
+        ],
+        "psr-4" : {
+            "Sabre\\HTTP\\" : "lib/"
+        }
+    },
+    "config" : {
+        "bin-dir" : "bin/"
+    }
+}