Add wrappers for Weka classifiers

Adds ConfidenceFunction subclass WekaConfidenceFunction
Adds ConfidenceTrainer subclass WekaConfidenceTrainer

Weka is GPL3, so this uses GPL3 License - did I set it up right?

Supports saving classifiers (e.g. WekaConfidenceFunction.save) but there is no corresponding support for deserializing a WekaConfidenceFunction - at least not yet. I could do that here, or we could make it a separate issue.

Let me know what you think!