[MemorySpool] Cloning messages to the queue

This PR is a merge of #278 and #518. With help from @xdecock we got the cloning to work. 

Can someone tell me why I get error running the `Swift_MessageTest`. 
I did not think the `require_once` was needed. But when I remove them I get the error: 
```
Class 'Swift_Mime_SimpleMessageTest' not found
```