Series.shift speed up
Series.shift() is slow when freq parameter is supplied

```
ts = pd.util.testing.makeTimeSeries(10000)

ts.shift(5, 'D') # takes 0.5 sec

ts.shift(5, np.timedelta64(1, 'D')) # takes 0.16 sec

ts2 = ts.copy()
ts2.index = ts.index + np.timedelta64(5, 'D') # takes < 0.001 sec
```
