ID3 and random decision trees and random forests

This patch
* Adds the `meta` sub-package which implements bagging
* Adds `RandomForest` in the `ensemble` sub-package
* Adds `ID3DecisionTree` and `RandomTree` to the `trees` sub-package

[`ID3DecisionTree` performs similarly to the reference](http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.52.8372&rep=rep1&type=pdf), `RandomForest` performance is competitive with (but a little lower than) the equivalent WEKA classifier.