Added incremental regression tree learner

Summary of the changes:
- added incremental regression tree learner;
- added two discretization techniques for regression (extended binary search tree and histogram-based);
- exposed the learner via JS API;
- added several classification and regression examples for the HoeffdingTree algorithm;
- stylistic changes and several bug fixes.

Notes:
- datasets are missing (too large to commit);
- in very near future, tree learners will also take feature vectors on the input.