Give github some time to notify that PR has been merged

I think one little `time.sleep` here could save many polling requests, while human's patience wouldn't get hurt. Let's say ordinary request to GitHub API lasts around...

```python
>>> from time import time
>>> start = time(); requests.get('https://api.github.com/users/octocat/orgs'); print time() - start
<Response [200]>
0.548367977142
```

...half second. So in the worst case, bb would perform 30/0.5=60 requests. Maybe I am missing something, it's Friday evening and I am tired :)