Support routing text messages outside of a call.

Asterisk now has protocol independent support for processing text messages
outside of a call.  Messages are routed through the Asterisk dialplan.
SIP MESSAGE and XMPP are currently supported.  There are options in sip.conf
and jabber.conf that enable these features.

There is a new application, MessageSend().  There are two new functions,
MESSAGE() and MESSAGE_DATA().  Documentation will be available on
the project wiki, wiki.asterisk.org.

Thanks to Terry Wilson for the assistance with development and to David Vossel
for helping with some additional testing.

Review: https://reviewboard.asterisk.org/r/1042/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@321546 65c4cc65-6c06-0410-ace0-fbb531ad65f3