Moved any knowledge of gtfs file names to GtfsDataProvider.
Added GtfsProviderSingleRowMock to provide single rows of data from each of the file types.
Including .txt files for these too.
Started work on python data model for basic gtfs objects. Base and Route objects added.
Added typeconverters.py. This will contain utilities to make sure
the values in the gtfs data model objects will be of the correct type.
Added short testing instructions to readme.