poor performance on Covertype Data Set

[wlattner]

Classification accuracy on the Covertype Data Set is less than 90% unless max_features is increased over the default sqrt(# features).

[wlattner]

This dataset has many constant features, such as the the soil type columns. When using only max_features of n_features, we need to make sure we visit at least one non-constant feature, otherwise too many nodes will be marked as leaf nodes. See 80c2e8f.