Cache control for now playing information

[jjst]

Some radio websites/players indicate start/end times or durations.

• Make start/end times part of the response
• Use it to provide cache control headers

[jjst]

Since #111 the start_time/end_time is not part of the response. This should now be trivial to implement: of all items return, take the one with the shortest end_time if one exists, and if so, populate:

Cache-Control: max-age=<seconds>

according to end_time - now(). https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control

Up to the client if they want to use the header to cache the response, but in any case if they call sooner the API should return a cached response.