boxplot: by + return_type='both' fails
I have a dataframe with two columns. When I try:
`temp.boxplot(sym="k.", by='column_1', return_type='both')`

I get:

```
> boxplot(data, column, by, ax, fontsize, rot, grid, figsize, layout, return_type, **kwds)                                                                                                                                                     
   2369         elif return_type == 'both':
   2370             ret = dict((k, BP(ax=ax, lines=line)) for
-> 2371                        (k, line), ax in zip(d.items(), axes))
   2372     else:
   2373         if layout is not None:

TypeError: izip argument #2 must support iteration
```

This is with `master`
