Output date-only string in BB JSON

This is an attempt to address the issue around dates @kachok raised here: https://github.com/blue-button/bluebutton.js/pull/92#issuecomment-41463210 while also making the API technically more accurate and making it possible to run our test suite in any timezone, not just PST (I hadn't even realized that was true!)

It also fixes a couple other issues I noticed while trying to get the test suite working.