Add "Exception day" feature

Problem/Motivation

I think something like exception day can be great idea.

For example, let's say we are open every Sunday, except on 24-dec (X-mas) every year.
User should select exact days (date from calendar, with repeat option) when the office is closed.

It can be valuable for most of us.

User interface changes

Data model changes

The 'day' column now has value 0-6 for the weekdays.
It will now also be used as a 'timestamp' to hold calendar dates, since this fits in the INT11 field type.

Here is a discussion on schema.org regarding both: https://github.com/schemaorg/schemaorg/issues/245:

So here is a proposal that addresses the above scenario:
- make it explicit that openingHours captures the general opening hours
- make it explicit that openingHoursSpecification captures general and/or explicit opening hours