This commit add Calendaring support to Asterisk for iCalendar, CalDAV, and MS
Exchange calendars. Exchange support has only been tested on Exchange Server 2k3
and does not support forms-based authentication at this time (patches *very*
welcome). Exchange support is also currently missing the ability to return a
list of a meting's attendees (again, patches are very, very welcome).

Features include:
  Querying a calendar for events over a specific time range
  Checking a calendar's busy status via the dialplan
  Writing calendar events via the dialplan (CalDAV and Exchange only)
  Handling calendar event notifications through the dialplan

(closes issue #14771)
Tested by: lmadsen, twilson, Shivaprakash

Review: https://reviewboard.asterisk.org/r/58


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@197738 65c4cc65-6c06-0410-ace0-fbb531ad65f3