unify API in regards to column-vs-columns
This is the "fix" only for 1 case out of few other possible:

```
$> git grep 'def.*column=' 
pandas/core/frame.py:    def boxplot(self, column=None, by=None, ax=None, fontsize=None,
pandas/stats/misc.py:def percentileRank(frame, column=None, kind='mean'):
pandas/tools/plotting.py:def boxplot(data, column=None, by=None, ax=None, fontsize=None,
```
