Add ability for server to check last message sender

As a server I want to know what peer has sent me a message. To do this all we have to do is
```
self._servers.get(name).get_peer_address()
```

It would be nice to have this data in message object so that we could check against some value
```
${msg} =  Server receives message
Should be equal as strings    ${msg.peer.ip}    127.0.0.1
```