Add event connections to attendees types.

[justlevine]

Adds event connections to the various attendee types. Closes #24 .

1. The Event type has rSVPAttendees, payPalAttendees, and wooAttendees.
2. rSVPAttendees, payPalAttendees, and wooAttendees can now be filtered by where : { eventsIn: $id }.

Note:

• Separate class-{type}attendee.php and class-{type}attendee-resolver.php files were created for each attendee type. This isn't very DRY, but was the best I could do without fully changing attendee over to an interface.
• The old class-attendee-resolver.php was never actually included before. There's a code segment from @kidunot89 that I left in, because I'm not really sure of the purpose/its effect. If it isn't necessary, it should be removed.
• For some reason, composer dump-autoload was generating platform_check.php. I manually removed it, but to be safe, a maintainer should probably regenerate the autoload.

[justlevine]

Requested changes merged.

[kidunot89]

@justlevine @bordoni This functionality was implemented in #46