Add proof of concept sklearn estimators.

This issue is mostly to track progress of the scikit-learn wrappers, but any feedback on me doing something stupid is obviously welcome.

Todo:

* [x] single classifier
* [x] classifier evaluator
* [ ] add simple evaluation metric (``accuracy_score``)
* [ ] add roc curve plotting
* [ ] add ``auc_roc_score``
* [ ] cross_val_score
* [ ] GridSearchCV
* [ ] Pipeline
* [ ] single regressor
* [ ] single transformer
* [ ] auto-discover classifiers / regressors
* [ ] auto-discover metrics
* [ ] optional evaluation of decision_function vs caching.