groupby boxplot creates more subplots than are needed
When calling `boxplot` on a `DataFrameGroupBy` object, it does not seem to be able to properly infer the number of subplots required to account for the number of groups. For example, the following:

```
lsl_dr[lsl_dr.test_type==t].groupby('type').boxplot(column='score', grid=False)
```

generates this plot:

![bad boxplot](http://d.pr/i/YReI+)

Notice that it generates a 2x2 grid, even though there are only three groups. I thought I could deal with this manually by calling `pyplot.subplots(1,3)`, but the `boxplot` method does not appear to be able to accept an axis argument like the `DataFrame.boxplot` method does. Is there a work-around for this?
