Add helper function for creating services (#39)

Author: robwittman

src/Api.php

@@ -2,6 +2,7 @@
 
 namespace Shopify;
 
+use Shopify\Service\AbstractService;
 use Shopify\Storage\PersistentStorageInterface;
 use Shopify\Storage\SessionStorage;
 use Shopify\Helper\OAuthHelper;
@@ -145,6 +146,17 @@ public function getOAuthHelper()
         return new OAuthHelper($this, $this->getStorageInterface());
     }
 
+    /**
+     * Helper function to create new service instances
+     *
+     * @param $serviceClass Fully Qualified Service className
+     * @return AbstractService
+     */
+    public function createService($serviceClass)
+    {
+        return new $serviceClass($this);
+    }
+
     public function loadApiKeyFromEnv()
     {
         $this->api_key = getenv(self::SHOPIFY_API_KEY_NAME);