GTFS graph and other new features

[irees]

• Add GTFS graph and graph loader
• Remove entity attribute prefixes
• Fixes to unicode/byte-order-mark handling
• Index and entities by ID
• Add 'id' and 'name' methods to entities with primary keys
• Shape lines (shapes.txt rows merged by shape_id and ordered by shape_pt_sequence)
• Add ServicePeriod entity, which merges calendar.txt and calendar_dates.txt service_id's
• Add trip_chunks method to create 'batches' of StopTimes
• Add stop_time_pairs method to create trip stop-stop edges
• Add pry dependency and "rake console" task

Note: this fork breaks backwards compatibility by dropping entity attribute prefix removal.