Issue passing a Timestamp to Series.between_time()
Hi,

I have an issue with passing pandas.Timestamp objects as arguments to pandas.Series.between_time(start, end).

It appears that pandas.tseries.index.DatetimeIndex.indexer_between_time() tries to convert start_time and end_time arguments as strings. 

The following lines give a temporary fix:

```
if not isinstance(start_time, Timestamp):
        start_time = to_time(start_time)
if not isinstance(end_time, Timestamp):
        end_time = to_time(end_time)
```

Thanks,
Simon
