-Change: inheritance of Meta options wasn't fully working.
-Change: Emit a warning, but don't stop models from being created.

Mind though, that the 2nd model will still not 'override' the first
without the override meta option.
-Add: Constants file.
-Change: rework the validation logic a bit so that the user can define special validators.
-Change: Let StringField trim the length if required.
-Add: Some basic validators.
-Add: Import validators and constants in __init__.py.