GTFS-RT

This is the major rewrite of the GTFS-RT code I've been working on for the past few weeks. It consists of two main changes:
1. The various subclasses of TripTimes are removed in favor of storing up to four arrays of stop times (arrivals, departures, scheduled departures and scheduled arrivals).
2. The KV8 remnants are removed in favor of direct processing of GTFS-RT TripUpdate objects.

Since this is a huge change, it's probably pointless for me to try to enumerate and explain all the little changes, so you can just ask questions if something's not clear. Major assumptions were documented, but I can imagine not everything is crystal clear yet.

This code needs lots of real-world testing. It seems to work just fine so far (although it rejects quite a substantial portion of the updates it receives, but this may be for good reason), but I don't dare to go anywhere near production with this code just yet.

This pull request is also intended (but not claimed) to address the following issues:
#1222
#1225