Using pandas.TimeSeries_DateFormatter in bar plot?
xref #5774

When time series is set as index, line plot uses the TimeSeries_DateFormatter for recognizable date formatting. However, bar plot use the FixedFormatter which outputs the datetime as it is. Can bar plot also use the DateFormatter?

```
In [1]: from pandas import *
In [2]: from numpy.random import *
In [3]: ts = Series(randn(100), index=date_range('1/1/2000', periods=100))
In [4]: df = DataFrame(randn(100, 4), index=ts.index, columns=['A', 'B', 'C', 'D'])
In [5]: df = df.cumsum()

In [7]: a = df.plot(kind='bar')
In [9]: a.get_xaxis().get_major_formatter()
Out[9]: <matplotlib.ticker.FixedFormatter instance at 0x108700f38>

In [11]: a = df.plot()
In [12]: a.get_xaxis().get_major_formatter()
Out[12]: <pandas.tseries.converter.TimeSeries_DateFormatter instance at 0x10877ccf8>
```
