[Decision Trees] Generic rescaling of predictions when constructing evaluator

Summary:
Thus transparent to clients what scaling should be applied for a given tree
- completely encapsulated by the factory function returning an evaluator.

Test Plan: unit tests.

Differential Revision: http://phabricator.tullo.ch/D37
[Decision Trees] Incorporate `go vet`, fix minor bug

Summary: Minor fix to annotations

Test Plan: unit tests

Differential Revision: http://phabricator.tullo.ch/D38