make template, content property can be callable (#132)

src/Message.php

@@ -80,7 +80,7 @@ class Message implements MessageInterface
      */
     public function getContent(GatewayInterface $gateway = null)
     {
-        return $this->content;
+        return is_callable($this->content) ? $this->content($gateway) : $this->content;
     }
 
     /**
@@ -92,7 +92,7 @@ class Message implements MessageInterface
      */
     public function getTemplate(GatewayInterface $gateway = null)
     {
-        return $this->template;
+        return is_callable($this->template) ? $this->template($gateway): $this->template;
     }
 
     /**