Support transit agencies using GTFS-realtime (GTFS-RT) via SIRI 2.0

According to our agencies table, both Ride-On and VRE offer GTFS realtime arrival information.

GTFS-RT is growing in popularity. We should support both of these agencies and any others using it.

ART also offers GTFS-RT, which might allow us to get rid of support for the Connexionz API.

Actually using GTFS-RT in TransitScreens will require some kind of other service offering a transactional API, such as a local installation of OneBusAway. See http://greatergreaterwashington.org/post/17460/whats-up-with-nextbus-part-3-where-ride-on-is-the-leader/ for some of the issues involved.