[WIP] Implementing Averaged SGD

Working on implementing averaged SGD for both classifier and regressor for #543
@dsullivan7 right, this way one should be able to implement partial fit

regarding ``(t' - t)``: I disagree, any mutation you make to the weight vector will be part of the averaged weight vector for the remaining iterations. The next time you encounter a non-zero value at ``j`` is just another mutation that you account for.