[composer] add autoload information

composer.json

@@ -34,5 +34,8 @@
         "symfony/twig-bridge": ">=2.0",
         "symfony/validator": ">=2.0",
         "twig/twig": ">=1.2.0"
+    },
+    "autoload": {
+        "psr-0": { "Silex": "src/" }
     }
 }