correctly updates known classes after teaching estimators

[philipjhj]

An attempt at solving issue #62.

Changing _set_classes to use other information in the learners (e.g. the labels of the given training data) than the classes known to the estimators broke tests, and was much more involved than moving the call to _set_classes into a local teach method to ensure the update happens after the training of the estimators.

Unsure whether the whole description of the teach function should be repeated.