nurse shark, first draft

Open for feedback, but *not* for merge. I'll be working on this branch as well to update the viz code for the new data model.

The only things you can test here presently are validation with the node script `dev/validate-with-timing.js` and the tests for nurse shark, `test/nurseshark_test.js`. The visualization itself is broken, as expected.

@cheddar I added a couple of things that were missing to the schematron, as per my comment on the Trello card.
Yeah, see my comment over [here](https://github.com/tidepool-org/blip/pull/130#issuecomment-57404371). I'm assuming we're not supporting that use case for now, but eventually I see passing nurseshark and TidelineData your preferred units and that will override what both "infer" about your units from the data.

The reason we're not supporting it now is because we don't have anywhere in blip for someone like you to specify which units you want to view your mixed data in, and I don't feel like a default is very appropriate in such a case (because which would we impose as the default, the American units or the international ones?)