Fix "$query" KeyError in with PyMongo 2.2+

This appeared to be happening in the case where there were no PyMongo queries
and an empty dictionary was returned.  Using dict.get fixes this problem, and
is backwards compatible with PyMongo < 2.2.

Fixes #13

debug_toolbar_mongo/operation_tracker.py

@@ -168,7 +168,7 @@ def _cursor_refresh(cursor_self):
         # Normal Query
         query_data['skip'] = privar('skip')
         query_data['limit'] = privar('limit')
-        query_data['query'] = query_son['$query']
+        query_data['query'] = query_son.get('$query')
         query_data['ordering'] = _get_ordering(query_son)
 
     queries.append(query_data)