add comment about Apache FastCGI workaround
use local user's time rather than web server time

This should ensure that the user usually gets the geohash for the current
local date, no matter in what timezone the server is located.

The user time is estimated from the longitude specified by user. The
estimate calculation was taken from http://www.geohash.info/srv/feed.phps.
improve error handling: return empty feed if DJI value is not available

This can happen if the user explicitly supplies a date in the future or
distant past, or if the date calculation logic has a bug :-)
Not sure how to handle these cases correctly, but HTTP errors don't seem
appropriate, so let's rather return an empty feed.
check that retrieved DJI value is actually a number
raise HTTP error 404 if user-supplied coordinates are out of range