DOC: Added a script demonstrating the use of the SKLLearnerAdapter class...

This is the counterpart of Yesterday's skl_trasformer_demo script. 
It tries to follow the same style/layout. The text will surely need some
further polishing.  Biggest questionmark: what skl object do we wrap
(estimator and/or predictor)? Learner seems to be gone, but I kept
the name for now because that's what the wrapper class is named.