Added 80% confidence intervals, some other minor enhancements

Feel free to merge this, but I'm going to keep working in this branch to ~~add charts to the details page.~~ done!

@steverit It'd be awesome if you could verify my math here. Just a simple standard error function testing for 80% confidence (chosen because that's what other A/B test suites use), so it should be all set.

ref #55