Timezone info is lost when pickling datetime indices
``` python
>>> c.head(3)
                                                           end_time
location_id sensor_id view_id start_time
54          1000305   0       2014-07-21 07:00:00+00:00    2014-07-21 07:15:00+00:00
                              2014-07-21 07:15:00+00:00    2014-07-21 07:30:00+00:00
                              2014-07-21 07:30:00+00:00    2014-07-21 07:45:00+00:00

>>> c.to_pickle(r"C:\temp\fff")
>>> pd.read_pickle(r"C:\temp\fff").head(3)
                                                   end_time
location_id sensor_id view_id start_time
54          1000305   0       2014-07-21 07:00:00  2014-07-21 07:15:00+00:00
                              2014-07-21 07:15:00  2014-07-21 07:30:00+00:00
                              2014-07-21 07:30:00  2014-07-21 07:45:00+00:00
```

Please note that the timezone is lost from `start_time`.
