Implement container-interop

This proposal is a simple implementation of ContainerInterface.
Now we can use it in
[zend-stratigility-skeleton](https://github.com/container-interop/container-interop) for example.

Author: Gianluca Arbezzano

composer.json

@@ -13,7 +13,8 @@
         }
     },
     "require": {
-        "php": ">=5.5"
+        "php": ">=5.5",
+        "container-interop/container-interop": "*"
     },
     "require-dev": {
         "zendframework/zend-di": "~2.5",

src/ServiceManager.php

@@ -9,7 +9,9 @@
 
 namespace Zend\ServiceManager;
 
-class ServiceManager implements ServiceLocatorInterface
+use Interop\Container\ContainerInterface;
+
+class ServiceManager implements ServiceLocatorInterface, ContainerInterface
 {
     /**@#+
      * Constants