Fare_Url to agency file

[KevinChicago]

In order to address some of the difficulty trip-planners, such as
Google Transit, have in dealing with the complexity of various fare
systems around the world, I suggest that the Agency.txt file have a
fare_url field added, which would allow the service provider to add a
url of a customer-facing webpage that describes that agency's various
fare and fare policies. Google could integrate as they see fit, but
hopefully would use the page as a replacement for situations where the
GTFS currently doesn't support a system's fare structure. It also
gives the transit agency to advertise other fare options (tourist
passes, reduced fares, weekly passes, etc.).

Kevin