Harmonize and reassess format of date constraint parameters

[kklein]

An example of such a parameter would be add_date_min_constraint and its min_value parameter.

First, it should be clarified, what currently works. Exemplary questions to ask:

• Can timestamps be used, as opposed to dates?
• Can the parameters strictly only be strings or do datetime objects with appropriate __str__ methods work as well?
• Is the currently suggested format, e.g. “‘20121230’”, the only one working?

Second, it should be asked how it should work. Some questions to ask:

• What are the respective ISO standards?
• What formats to which databases support?