remove expires, rework comment a bit
use default caching from express

Tested using curl, sending the if-modified-since header, and monitored
using chrome.

Reloads sources when they have changed, otherwise allows using cached
version.
show correct time when brushing

When adding retrospective, the brushing extent should factor in the 30
minute offset when displaying "now".
remove dead code
re-work audio handling

This allows the browser to cache things better.
Re-uses same media object across alarms.
Ensure that audio loops once it starts
Merge pull request #4 from nightscout/express-cache

Express cache