Batch API

This pull request isn't complete -- it needs more unit tests.

That said, I think the JS changes are ready for review. Overview:
* Add a new dict to keep track of which stats data's been retrieved/in the process of retrieval (so we don't try to retrieve it again)
* All requests for stats data (which is currently organized by state/county) get sent to the "batch" api, which handles the responses in one swoop