API: Change Panel.shift arg signature to match generic's
This surfaced in the Panel `pct_change` implementation at https://github.com/pydata/pandas/pull/6909

`generic`: `def shift(self, periods=1, freq=None, axis=0, **kwds):`
`Panel`:    `def shift(self, lags, freq=None, axis='major'):`

I had to adjust the call to `shift` in `generic.pct_change` to not use a kwarg for `lag`/`period`:

Before: `rs = data / data.shift(periods=periods, freq=freq, **kwds) - 1`
After: `rs = data.div(data.shift(periods, freq=freq, **kwds)) - 1`

We'll need to keep the `.div` change, but we can change back the kwarg once the signatures are aligned.
- [x] Add kwarg deprecation decorator
- [x] change `pct_change` back to kwarg
