[WIP] Implementing Averaged SGD

Working on implementing averaged SGD for both classifier and regressor for #543
Alternatively, we could reuse the pure-python, simple but provably correct [perceptron implementation in the tests](https://github.com/scikit-learn/scikit-learn/blob/master/sklearn/linear_model/tests/test_perceptron.py) to add averaging and check that the results are the same.