Adds fetchTransaction

As a proxy to `FindRequest`. Functionality already exists with `FindRequest` but `fetchTransaction` method provides interface consistency with other gateways.

Author: drewm

src/Gateway.php

@@ -229,4 +229,13 @@ public function void(array $parameters = array())
     {
         return $this->createRequest('\Omnipay\Braintree\Message\VoidRequest', $parameters);
     }
+    
+    /**
+     * @param array $parameters
+     * @return Message\FindRequest
+     */
+    public function fetchTransaction(array $parameters = array())
+    {
+        return $this->createRequest('\Omnipay\Braintree\Message\FindRequest', $parameters);
+    }
 }