trees: Handling FloatAttributes.

This patch adds:

	* Gini index and information gain ratio as
           DecisionTree split options;
	* handling for numeric Attributes (split point
           chosen naïvely on the basis of maximum entropy);
	* A couple of additional utility functions in base/
	* A new dataset (see sources.txt) for testing.

Performance on Iris performs markedly without discretisation.
Merge pull request #91 from Sentimentron/numeric-staging

trees: Handling FloatAttributes.