[WIP] Implementing Averaged SGD

Working on implementing averaged SGD for both classifier and regressor for #543
Also, to allow for partial fit, wouldn't it be possible to just scale the average weight vector by ```t_``` (the total number of iterations) before the iterations of samples begin? That would give the sum of all weights up to that point and you can proceed as if you were computing the average. At the end, you would compute the average by dividing by ```t_```.