robust-gtfs

**Making `AB_GTFStoSQL` robust to omissions in GTFS feeds**

Some columns in the GTFS are optional. So far, `AB_GTFStoSQL.py` has assumed that the provider of the feed has supplied empty columns for missing/null information. This is not always the case: some feeds only give the required fields, and do not have empty records for missing optional data. This branch correctly behaves as if no column headings are given, and avoids relying on a consistent column order.

Only additional module is `os.path`, because it also makes the allowance that some entire files (I'm looking at you, `shapes.txt`) are optional, too.