shift(..) applied to datetime series
I am reporting a bug in which a TimeSeries containing datetime values are converted to float64 values upon application of the shift(self, offset) function.  Version 0.10.1, python 2.7.

As an example below, I excerpted the first ten rows of end_time from the data frame listed.  The dtype of this timeseries is datetime64[ns].  When I apply shift(2), the values are converted to float64.  I tried applying the convert_type and astype functions to this timeseries, but neither would return the timeseries to its original datetime64[ns].

Expected behavior: data type of time series should not change upon shifting.  

```
DatetimeIndex: 88449 entries, 2012-09-04 07:03:00 to 2013-02-25 20:00:00
Data columns:
time        88449  non-null values
end_time    88448  non-null values
mid         88449  non-null values
dtypes: datetime64[ns](2), float64(1)
```

```
In [70]: end_times
Out[70]: 
time
2012-09-04 07:05:00   2012-09-04 07:06:00
2012-09-04 07:06:00   2012-09-04 07:07:00
2012-09-04 07:07:00   2012-09-04 07:08:00
2012-09-04 07:08:00   2012-09-04 07:09:00
2012-09-04 07:09:00   2012-09-04 07:10:00
2012-09-04 07:10:00   2012-09-04 07:11:00
2012-09-04 07:11:00   2012-09-04 07:12:00
2012-09-04 07:12:00   2012-09-04 07:13:00
2012-09-04 07:13:00   2012-09-04 07:14:00

In [71]: end_times.shift(2)
Out[71]: 
time
2012-09-04 07:05:00             NaN
2012-09-04 07:06:00             NaN
2012-09-04 07:07:00    1.346742e+18
2012-09-04 07:08:00    1.346742e+18
2012-09-04 07:09:00    1.346742e+18
2012-09-04 07:10:00    1.346743e+18
2012-09-04 07:11:00    1.346743e+18
2012-09-04 07:12:00    1.346743e+18
2012-09-04 07:13:00    1.346743e+18
Name: end_time
```
