Support value types other than f32

[robsmith11]

Correct me if I'm wrong, but it appears that for both features and classes in the fitting of decision tress, rustlearn only supports arrays of f32 values. Is there a reason for this limitation? I would think that features simply need to have the Ordering trait and classes have the Eq trait. It would be useful to have at least f32, f64, i32, i64, and bool I think.