sharex and sharey do not turn off redundant tick labeling
sharex and sharey do not turn off redundant tick labeling, is this an intended deviation from matplotlib `subplots()` behavior?

[pandas.tools.plotting._subplots()](https://github.com/pydata/pandas/blob/master/pandas/tools/plotting.py#L923) is a copy from matplotlib, but the code to remove the redundant tick labeling when using _sharex=True_ or _sharey=True_ has been left out. 
This is the code present in matplotlib  `subplots()` but not in [pandas.tools.plotting._subplots()](https://github.com/pydata/pandas/blob/master/pandas/tools/plotting.py#L923)

``` python
    # turn off redundant tick labeling
    if sharex and nrows>1:
        # turn off all but the bottom row
        for ax in axarr[:-1,:].flat:
            for label in ax.get_xticklabels():
                label.set_visible(False)


    if sharey and ncols>1:
        # turn off all but the first column
        for ax in axarr[:,1:].flat:
            for label in ax.get_yticklabels():
                label.set_visible(False)
```
