TNL-301 Filter posts by group_id.

https://openedx.atlassian.net/browse/TNL-301

@gwprice @jimabramson Please review.
Without the sort, the following unit test fails (since we are not using sorted-dicts):
```
Traceback (most recent call last):
  File "/edx/app/edxapp/edx-platform/notifier/notifier/tests/test_pull.py", line 241, in test_empty
    p.assert_called_once_with(expected_api_url, headers=expected_headers, data=expected_post_data)
  File "/edx/app/edxapp/notifier-env/local/lib/python2.7/site-packages/mock.py", line 846, in assert_called_once_with
    return self.assert_called_with(*args, **kwargs)
  File "/edx/app/edxapp/notifier-env/local/lib/python2.7/site-packages/mock.py", line 835, in assert_called_with
    raise AssertionError(msg)
AssertionError: Expected call: post('*test_cs_url*/api/v1/notifications', headers={'X-Edx-Api-Key': '*test_cs_key*'}, data={'to': '2013-01-02 00:00:00', 'user_ids': 'a,b,c', 'from': '2013-01-01 00:00:00'})
Actual call: post('*test_cs_url*/api/v1/notifications', headers={'X-Edx-Api-Key': '*test_cs_key*'}, data={'to': '2013-01-02 00:00:00', 'user_ids': 'a,c,b', 'from': '2013-01-01 00:00:00'})
```