WIP: PU learning: the 1-DNF method

Early pull request: the 1-DNF method for binary classifier learning from positive and unlabeled data only (PU learning). Issues:

* this works more or less on the dataset I'm currently working on, but that's not public yet, huge and XML-encoded
* I tried to come up with an example based on 20news (my `pu-learning-example` branch), but that performs very poorly
* the implementation is a transformer that takes `X` and `y` and returns `y_transformed` rather than `X_transformed`; this is different from other transformers
* I've adopted the convention that `-1` means unlabeled

If someone can devise an example that makes this work on a benchmark dataset, I'd be very glad...