[MRG] Add quantile strategy to DummyRegressor. Fixes #3421.

> Would you like me to add support for sparse y to DummyRegressor?

At the moment, regression won't support sparse output matrices as they are dense by nature. Maybe my remark was not judicious, if you find it better you can use `len or y.shape[0] instead.

> In addition to the above, what do you think of allowing an empty 'y' when the 'constant' strategy is used?

This would make the dummy classifier and regressor a special case. I would avoid having exception. It doesn't make sense to fit a classifier on zero sample.