Hide zone labels

@dragonfi Another last-minute request, this time from a edX Solutions client, to an option to hide the zone labels, and remove the transparency on the popups for increase visibility. Also pinging you on this since you were the last to make changes.

I have also fixed the tests, which were failing with:

```
1) ERROR: test_drag_and_drop_v2.test_ajax

   Traceback (most recent call last):
    /home/antoviaque/.virtualenvs/edx-xblock-dragndropv2/local/lib/python2.7/site-packages/nose/case.py line 197 in runTest
      self.test(*self.arg)
    tests/test_drag_and_drop_v2.py line 89 in test_ajax
      res = json.loads(block.handle('do_attempt', make_request(data)).body)
    /home/antoviaque/.virtualenvs/edx-xblock-dragndropv2/src/xblock/xblock/core.py line 293 in handle
      return self.runtime.handle(self, handler_name, request, suffix)
    /home/antoviaque/.virtualenvs/edx-xblock-dragndropv2/src/xblock/xblock/runtime.py line 648 in handle
      results = handler(request, suffix)
    /home/antoviaque/.virtualenvs/edx-xblock-dragndropv2/src/xblock/xblock/core.py line 122 in wrapper
      response_json = json.dumps(func(self, request_json, suffix))
    drag_and_drop_v2/drag_and_drop_v2.py line 198 in do_attempt
      'component_id': self._get_unique_id(),
    drag_and_drop_v2/drag_and_drop_v2.py line 239 in _get_unique_id
      unique_id = self.parent.replace('.',  '-')
   AttributeError: 'NoneType' object has no attribute 'replace'
```

I have also ensured the feedback popups are always on top of the dropped items (which wasn't the case due to the way z-index is reset when a containing element is set to be relative), and cleaned up some pylint warnings in the test file.

Can you review/test? Merging now to get it to QA, but still worth a careful review. Thanks!