Add ability to retrieve customData for groupMember

I think the data we are using is not generic so provide access to non-generic data

src/Responses/Contact.php

@@ -82,7 +82,7 @@ public function getOptOutIsoDateTime() {
    * @return mixed
    */
   public function getOptInSource() {
-    return $this->data['opt_in_details'];
+    return (string) $this->data['opt_in_details'];
   }
 
   /**
@@ -136,6 +136,10 @@ public function setContactReference($contactReference) {
     $this->contactReference = $contactReference;
   }
 
+  public function getCustomData($key) {
+    return (string) $this->data[$key];
+  }
+
   /**
    * @var  SilverpopConnector
    */