Add HTTP caching info based on revision dates

[Cantido]

HTTP responses from the API return cache-control and last-modified headers based on the revision dates provided in the Telnet responses. Example:

horizons/test-resources/earth-geophysical.txt

Line 3 in f782558

Revised: Jul 31, 2013 Earth 399

For an ephemeris request submitted without options, caching must be disabled, or restricted to whatever amount of time the request is still valid. I think the system starts its two-week window at the beginning of the current UTC day, so the response could at least be cached until the end of the day.