Add TransXchange convertors

[barbeau]

See https://groups.google.com/forum/#!topic/transit-developers/TGJH-gBLYGA. Creating this as a placeholder issue so I don't forget :).

[luqmaan]

Which links should we add?

• https://github.com/Transporter-Project/transxchange-to-gtfs
• https://github.com/jpf18/TransXChange2GTFS
• https://www.gov.uk/government/publications/transxchange-publisher

[barbeau]

I'd say all of the above. Seems like there isn't a single winner at this point and they could all use work.