Fix issue when new keys are available in datasource (#21)

Fix issue when new keys are available in datasource

_states/grafana3_datasource.py

@@ -254,7 +254,9 @@ def _diff(old, new):
     for key in old_keys:
         if key == 'id' or key == 'orgId':
             del old[key]
-        elif old[key] == new[key]:
+        # New versions of Grafana can introduce new keys that are not present
+        # in _get_json_data.
+        elif key in new and old[key] == new[key]:
             del old[key]
             del new[key]
     return {'old': old, 'new': new}