Panel pct_change bug
```
  File "z:\code\m.py", line 420, in get_panel
    self.source_panel.pct_change()
  File "c:\anaconda\lib\site-packages\pandas\core\generic.py", line 3418, in pct_change
    rs = data / data.shift(periods=periods, freq=freq, **kwds) - 1
TypeError: shift() got an unexpected keyword argument 'periods'
```

> pandas.version.version
> Out[5]: '0.13.1'
