Basic implementation of eventinfo and checkin

This will not fly as it is, but should work as a starting point for discussing it. My questions:

- Is this a good way to group together eventinfo with the checkin functionality?
- Shouldn't we make some changes so that API calls that modify data POST instead of GET?
- How should the parameters work for choosing event and user? What should the defaults be?
- Is this a reasonable way to create the response object? My argument for it is that it makes a bit clearer response with correct JSON value types.

If we decide to carry on with this, `checkin.php` is obsolete of course.