ENH: Allow use of keep="all" in duplicated
#### keep="all" as parameter option in `duplicated`

My view is that a more intuitive parametrization of the `duplicated` function would be that `keep='all'` should allow you to keep all, since `keep=False` kind of doesn't make any sense.

I have started on a pull request but had not realised how tightly coupled the `drop_duplicates` and `duplicated` functions are (but it makes sense of course) - I guess this is why the choice of `keep=False` was made in the first place.

I am of the view that `keep="all"` in the `drop_duplicates` method should raise an error, because again, that would be counter-intuitive behaviour.
