Sampling, batch_all, non-zero, optimizer-flag

[lucasb-eyer]

So this is a branch I had lying around for a while which adds a lot of things. I'm not merging it yet, as I still want to add these changes to the README and test it thoroughly, but here it is for others to try out.

It implements the trick I mentioned in #4 as well as some more variants we cover in the paper. If you give this a try, please give feedback here.