[WIP] Implementing Averaged SGD

Working on implementing averaged SGD for both classifier and regressor for #543
I would rather say converge with fewer iterations over the training set.

Indeed if the users does not reduce `n_iter` when using ASGD, the training time will be longer training times (because of the averaging overhead) as we do not do any convergence checks in `SGDClassifier` / `SGDRegressor`.