Add thread id to debug logs

Makes it easier to troubleshoot S3 debug logs.

Logs look like:

```
2014-06-06 11:26:45,583 - MainThread - awscli.customizations.s3.executor - DEBUG - Queueing end sentinel for worker thread (priority: 1)
2014-06-06 11:26:45,583 - Thread-4 - awscli.customizations.s3.executor - DEBUG - Shutdown request received in worker thread, shutting down worker thread.
2014-06-06 11:26:45,583 - MainThread - awscli.customizations.s3.executor - DEBUG - Queueing end sentinel for worker thread (priority: 1)
2014-06-06 11:26:45,584 - MainThread - awscli.customizations.s3.executor - DEBUG - Queueing end sentinel for worker thread (priority: 1)
```

cc @danielgtaylor