Output prediction probs

[wonjininfo]

Added feature to output probability scores for test dataset. If output_prob flag is set, output file includes model's prediction probability columns.

[tmills]

We discussed in hackathon to refactor as an optional output_probs argument to write_predictions_for_dataset(), it should be False by default, give a warning for non-classification tasks, and just write a slightly different string if output_probs is true.