BUG: Make DataFrame not hardcode it's own constructor in the code. Issue #2859
BUG: Make DataFrame not hardcode it's own constructor in the code. Issue

Changed all occurrences of `return DataFrame` to `return
self._constructor` and changed the latter to return `self.__class__`
rather than `DataFrame` by default.

Also made static methods like `DataFrame.from_dict` work on the
`cls` passed to them rather than on hard-coded `DataFrame`.

This is a minimal quick win on the cases described in Issue #2859.
A formal approach to make classes friendly to subclassing might still be
needed, see discussion in the case.
