Adds Exception propagation so reconnect support works.

Propagates recoverable exceptions from the background thread to the Main Thread so that retry functionality works. The changes come with tests and can be run using:

```
nosetests --with-coverage --cover-html -v kombu.tests.transport.test_qpid
```

Also tested with Pulp, and all Celery use survived qpidd restarts in testing. See the docblocks for more info on the design.