Preserve stop sequence values from GTFS so they can be matched to updates in a GTFS-realtime feed.  Fixes #50.
Use MappingLibrary.mapToValue rather than a for loop.
Per feedback from @barbeau, rename originalSequence to gtfsSequence for clarity.
Document distinction between sequence and gtfsSequence in StopTimeEntry.
Merge branch '50-stoptime-indices' of https://github.com/kurtraschke/onebusaway-application-modules into develop