Add support for proposed route_sort_order field

This PR adds support for the `route_sort_order` field, [proposed for inclusion in the GTFS standard](https://groups.google.com/d/topic/gtfs-changes/73resybBoc0/discussion) and already in use in TriMet's [GTFS feed](http://developer.trimet.org/schedule/gtfs.zip).  This is the first step toward supporting `route_sort_order` in the various OneBusAway user interfaces.