demo code should be updated 

The code of demo should be :
```
token = harv.fetch_token("https://id.getharvest.com/api/v2/oauth2/token", client_secret="CLIENT SECRET", authorization_response=response_uri, include_client_id=True, state=state)
```

otherwise you will got exception says:
```
(invalid_request) 'client_id', 'client_secret' required
```

version: v2_dev