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

My intention is to include the endpoints 0 and 1, quantiles representing the minimum and maximum values. (This parallels np.percentile, which allows 0 and 100 representing the min and max.) In my experience [0, 1] represents an interval with the endpoints included. (http://en.wikipedia.org/wiki/Interval_(mathematics)). Could you describe why you suggest (0, 1)? Perhaps it is an internationalization issue?