Added a 'stop_prefix' field to the required config dict for each mode. Will
determine the prefix used in creating stop names in the GTFS. needs to match
up with the names used in the route segments topology file given as a
parameter to create_gtfs_from_basicinfo.py

Also, added details for a new mode, 'bus-motorway', to correspond to BZE's
latest work in proposing a network of high-speed motorway buses.
1) In create_gtfs_stop_entries(), made the stop name creation read and use the
'stop_prefix' parameter of the configuration for the mode being processed,
rather than just always using 'B' for buses. See previous commit.

2) Added better error-handling to save_seq_stop_speed_info(): now,
it will check and print a better error if fields relating to speed are missing
from the shapefile, if they are needed. If not, just continues.