partially rewritten field tests
working on error messages
polishing error messages
renamed verify -> check
copied model_errors
split invalid_models.invalid_models -> invalid_models.{old_invalid_models, invalid_models}

+ deleted unnecessary `override_settings` decorators for test cases
making tests running (failing instead of ending in errors)
added django.core.checks.{Warning, Error} classes
added Field.check() method
fixed bugs in tests
first tests pass (tests checking CharField.max_length attribute)

- Added Error.__eq__(self, other) method
- Changed behaviour of Field.check(self, **kwargs) -- added setting `obj`
  attribute for each error raised by `check_*` methods.
- Written CharField.check_max_length_attribute method.
- Deleted relevant tests from invalid_models.old_invalid_models module.
small improvements

- renamed _Error -> BaseError
- comparing BaseErrors checks `obj` fields
- rewritten some CharFieldTests -- added `obj` field to Error instances
rewritten checks of Field.choices
rewritten checks of Field.db_index
rewritten checks of DecimalField
rewritten checks of FileField.upload_to
rewritten checks of BooleanField.null
rewritten checks of GenericIPAddressField
small fixup -- rewritting checks of CharFields
added BaseError.__repr__ method