Add validation of date fields

This is a patch to add support for date field validation (equals, between, later than and prior to).

I wasn't sure what would be the best format to accept the arguments in timestamp, formatted date or a strtotime() parseable string, so I've gone with the latter for now, but that could be changed easily enough.

Can the default error messages take an error_callback like custom ones? If so I could look into writing one that handles the different cases.