[Decision Trees] [PB] Serialize metrics from training with model

Test Plan: unit tests

Differential Revision: http://phabricator.tullo.ch/D33
[Decision Trees] Compute learning curve metrics for progressive rounds

Summary: Progressively compute ROC/NE/LogScore for each round of boosting.

Test Plan: unit tests

Differential Revision: http://phabricator.tullo.ch/D34
[Decision Trees] Command-line wrapper for training models

Summary:
Give a JSON-encoded instance of training examples and a configuration file,
trains a model and dumps the trained model to stdout.

Test Plan: unit tests, run the tool.

Differential Revision: http://phabricator.tullo.ch/D35
[Decision Trees] [UI] Display ROC/LogScore/NE for models

Summary:
Displays ROC/LogScore/Normalized Entropy in a single graph with NVD3.js

Test Plan: unit tests