Moved all scheduling related functionality.

This adds a new plugin Sabre\CalDAV\Schedule\Plugin, and is the first
step towards calendar-auto-schedule support.
Added an inbox node.
Moved scheduling properties to the scheduling plugin.
Typo.
Correcting typo in getFeatures on auto schedule plugin

calendar-auth-schedule to calendar-auto-schedule
A bit more documentation.
Finding attendees.
Sending emails when inviting attendees.