Add to_dataframe() method to MultiIndex
I find the `Index.to_series` method is a convenient way to allow indices to act as columns of a dataframe where desired. However, the behavior of `MultiIndex.to_series`, which gives a `Series` of tuples, is less useful.

Would it be convenient to provide a `to_dataframe` method for index classes? This would be a natural extension of the utility of `to_series`, and more useful for `MultiIndex` objects I would think.

I'm   something equivalent to:

> ```
> def to_dataframe(self):
>    DataFrame(self.tolist(), columns=self.names, index=self)
> ```
