ENH: move trained_{targets,dataset,nsamples} from the Classifier into Learner

they could be generally useful, although some times some of them
are a bit too specific to classifiers, e.g. some beasts
could be trained on plain arrays, thus using now len() instead of nsamples
and checking of AttrDataset for fetching the targets

Closes #126

Short on time so sending out as a PR to see if any tests break ;)