Calendaring support for Exchange Server 2007+ via EWS

This commit adds support for calendaring with Exchange Server 2007+ via
Exchange Web Services. Full write support and for querying attendees. Many
thanks to Jan Kaláb for the feature.

(closes issue #17022)
Reported by: pitel
Patches: 
      res_calendar_ews.c uploaded by pitel (license 1008)
Tested by: pitel, twilson

Review: https://reviewboard.asterisk.org/r/557/
Review: https://reviewboard.asterisk.org/r/668/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@265317 65c4cc65-6c06-0410-ace0-fbb531ad65f3