TODO

* GTFS export from database.
* Eliminate dependency on SQLite; gtfs should work on any DB which SQLAlchemy
  supports.
* Make it possible to open more than one schedule at once; the following code
  currently doesn't work:

    from gtfs import Schedule
    from gtfs.entity import *
 
    sched1 = Schedule('bart.db')  #open first schedule
    print sched1.agencies         #works fine
    print Agency.query.all()      #also works fine

    sched2 = Schedule('nyct.db')  #open second schedule
    print sched2.agencies         #works fine
    print Agency.query.all()      #works, but gives results from sched2

  The problem is this line in Schedule:

    Base.metadata.bind = self.engine

  This creates a global shared state across the entity classes, so they are
  bound to the most recently instantited Schedule object

* Investigate using GeoAlchemy for real geospatial support?  GeoAlchemy plus a
  geo-enabled database might be too heavy a dependency for some cases; setup of
  PostGIS (or Spatialite, etc.) is non-trivial.