API: DataFrame constructor for subclassing Series

[sinhrks]

Series.to_frame uses pandas.DataFrame statically as frame constructor. It is nice if Series has overridable property to specify frame-like constructor, like DataFrame has _constructor_sliced.

How about Series._constructor_frame?

[jreback]

maybe _constructor_expanddim? should add to generic as NotImplemented; change to_panel (in frame) to use as well