Adds middleware locator

Based on the idea of the handlerLocator of Tactician this patch adds
a middleware locator. To make this package more independend of the other packages
of php-api-clients.

Author: jaapio

composer.json

@@ -17,6 +17,7 @@
   },
   "require-dev": {
     "api-clients/test-utilities": "^1.0",
+    "container-interop/container-interop": "^1.1",
     "guzzlehttp/psr7": "^1.3"
   },
   "suggest": {