aprs.fi API, optimisations and rate limiting

The programmers out there might want to try out the aprs.fi API. The code has been there for a while, but I've now polished and documented it a bit. As usual, feedback is welcome.

This evening I've added some optimisations which should make the site load faster, especially over slow connections. It will still be slow on slow mobile devices, I'm afraid.

I've also added rate limiting to protect the service against automated queries by unknown spiders and harvesters. Please let me know if it hits you during normal use. Right after it's installation on Tuesday afternoon it hit all users for a few minutes due to a little installation-time quirk, but that was, sort of, expected. :)