Add download_dataset function (and one do Do It Live!)

[jashapiro]

Closes #35

The main addition here is the download_dataset() function, which really just does what it says and should behave more or less like the download project function, but for taking a dataset... The main extra is that it has a check to be sure the dataset is actually ready before trying to download it.

I also added handling of expired datasets, since that is a thing that can happen.

Finally, I also made a function that waits until a dataset is ready and then downloads. I wasn't sure if this should really be a separate function, but the additional option made me think that it probably should be... But I could reconsider and roll them together, with a separate internal function that just does the status polling. Let me know what you think!

The biggest challenge here was deciding how often to poll and how long to wait: For now I set polling to every 30 seconds: In my testing, processing for small datasets takes somewhere between 30 sec and 1 min, so faster polling didn't seem worth doing. But I did leave it as an option in case people really want to poll at a different rate. I also got to play around a bit with cli to make a spinner while waiting. The elapsed time you get with cli is a bit disappointing in that you can't control rounding, unless you want to do all the seconds to minutes translations yourself, so I just left it as the default.

I'm also now allowing downloads without automatic unzipping, when ends up in a bit a strange place for the arg because I didn't want to break position-based calls any more than already.