Added some error checking for shapefile missing necessary stop IDs.
Added a list-parsing from cmd line func.
Two new files, starting to get a set of useful utilities for manipulating and
modifying an existing GTFS schedule:-
 * gtfs_ops.py :- is meant for re-usable functions such as copying all trips
   for an entire route etc.
 * A new cmd-line script to subset a GTFS file and just copy across entries
   for selected routes.