Delay survey submissions

For issue #37. This queues survey submissions and tries resending them if there's a problem with Internet connectivity. @christhompson had landed some starter code; this goes the last mile and gets it working.
I'm not quite familiar with indexedDB, but this code looks a bit odd. You assign request to indexedDB.open, add event handlers to it, but then you don't use it in the end. I'm surprised this actually works.
I kind of expected something like ```request.send()``` or ```return request;``` at the end.