WorkerQueue doesn't flush on exit

The following won't report anything when run as a script, but it will work when from REPL:

```
import errplane
_errplane = errplane.Errplane("errplane_api_key", "errplane_application_id")
_errplane.report("problem")
```

Seems like `WorkerQueue` doesn't get flushed before exit because it uses daemonized worker threads.

Cleanest solution in my mind is adding `Errplane.flush` that blocks.
