add http cache friendly headers

[gfldex]

I'm working on Perl 6 bindings for radiobrowser-api and found myself in need to fetch the whole list of stations. Waiting for 60+ seconds proved to be rather boring. Hence my humble request to add support for the http headers fields Last-Modified, If-Modified-Since and If-Unmodified-Since.

There is the problem of the click* fields in a response. So it may even make send to have a special api call that returns a timestamp for when a station was added/removed/broke or even a call that only returns changed stations since a given date. The latter could help a great deal to keep the api responsive even if those 1 billion netizen start to use your fine service every day.