[WIP] Implementing Averaged SGD

Working on implementing averaged SGD for both classifier and regressor for #543
In order to relax the requirement of knowing T, would it work to store two sparse vectors (same sparsity pattern as `aw`), one counting how many updates have been made to each variable, and the other counting surviving iterations? (Basically incrementing itself by the update-count vector.)

But if updates end up not being sparse, the overhead might become significant. So the impact of regularization might matter the most.