Add LOGOUT operation to IMAPDisconnectOperation and add ObjC API

RFC 3501 section 3.4 suggests clients should send LOGOUT before disconnecting. This patch adds the relevant libEtPan call and adds an Objective-C API to create an IMAPDisconnectOperation.

Seems ok here (in an operationQueueRunningChangeBlock):

```
2013-10-01 23:16:43:250 Spitfire[5218:a41b] Got 17 messages, deleted 0 from INBOX (ryan@testtoast.com)
2013-10-01 23:16:44:280 Spitfire[5218:700b] 60 LOGOUT

2013-10-01 23:16:44:291 Spitfire[5218:700b] * BYE Logging out
60 OK Logout completed.
```