Refactoring: add a new abstract class GtfsObject that
Agency and Stop extend.

the _readField() private method now belong to GtfsObject.
Add new test to test_stop: an exception is thrown if stop_id is missing.
Adda new test to test_stop: an exceptionis thrown if stop_name is missing.
Refactoring: use a for each loop to test the mandatory fields.
Refactoring: put the mandatory field within a static const list.
Refactoring: move _mandatoryFieldIsMissing() to GtfsObject.
Code formating: replace tabs by spaces.