Add Calendaring support for Asterisk

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