DataFrame.tz_localize
`tz_localize` is implemented in NDFrame. But, AFAICT, `tz_localize` doesn't and can't work on a DataFrame


```python
In [14]: df = pd.DataFrame({"A": pd.date_range('2000', periods=4)})

In [15]: df.tz_localize('UTC')
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-15-4390d0c58a16> in <module>
----> 1 df.tz_localize('UTC')

~/sandbox/pandas/pandas/core/generic.py in tz_localize(self, tz, axis, level, copy, ambiguous, nonexistent)
   9374             if level not in (None, 0, ax.name):
   9375                 raise ValueError("The level {0} is not valid".format(level))
-> 9376             ax = _tz_localize(ax, tz, ambiguous, nonexistent)
   9377
   9378         result = self._constructor(self._data, copy=copy)

~/sandbox/pandas/pandas/core/generic.py in _tz_localize(ax, tz, ambiguous, nonexistent)
   9354                     ax_name = self._get_axis_name(axis)
   9355                     raise TypeError('%s is not a valid DatetimeIndex or '
-> 9356                                     'PeriodIndex' % ax_name)
   9357                 else:
   9358                     ax = DatetimeIndex([], tz=tz)

TypeError: index is not a valid DatetimeIndex or PeriodIndex
```

Anyone know why? OK with removing?