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

Looks good, I have just thought to a simpler version based on yours.

What do you think of

```
if self.quantile is None or self.quantile < 0 or self.quantile > 1:
    raise ValueError("Quantile has to be specified in the range [0.0, 1.0], "
                     "but got %s." % self.quantile)
`````
?