SchemaDate min and max validators

Default error messages added, SchemaDate min and max validators added,
validation tests added.
Unfortunately, since this error message gets run through the regex at the time the schema is attached to the model the message would then show a static date and time. I wasn't sure what text to put here, since 'Date.now()' is specific to JavaScript. I was thinking of putting "Current UTC date", but I wasn't sure. Your thoughts?