Series.shift bugs, set default to periods=1
```
In [46]: ts[:5]
Out[46]:
2000-01-01 00:00:00    1.126328
2000-01-01 01:00:00   -1.551378
2000-01-01 02:00:00   -0.353657
2000-01-01 03:00:00    1.447303
2000-01-01 04:00:00   -0.423588

In [45]: ts[:5].shift(1, offset=pd.datetools.Hour(4))
/home/wesm/code/pandas/pandas/core/series.py:2517: FutureWarning: 'timeRule' and 'offset' parameters are deprecated, please use 'freq' instead
  FutureWarning)
Out[45]:
2000-01-01 01:00:00    1.126328
2000-01-01 02:00:00   -1.551378
2000-01-01 03:00:00   -0.353657
2000-01-01 04:00:00    1.447303
2000-01-01 05:00:00   -0.423588
```
