Added the twoing node splitting criterion. While it is most important to offer entropy and gini splitting criteria, it may be nice to offer twoing as well.

Reference: Breiman, L. Some Properties of Splitting Criteria, Statistics Department, University of California, Berkeley. 1992.

Added some very basic jUnit tests for the twoing computation.
Merge pull request #32 from ericeckstrand/feature-twoing-node-split-criteria

Well done! I tested this internally on your fork with some edits to expose TWOING to the REST API.

Merging this PR now.