Adds force encryption option to iax.conf

This patch adds forceencryption=yes as an iax.conf option.  When force encryption is enabled, no unencrypted connections are allowed.  This insures all connections are encrypted.  This is a new feature, so CHANGES and iax.conf.sample are updated as well.   

(closes issue #13285)
Reported by: sgofferj
Tested by: russell
Review: http://reviewboard.digium.com/r/150/



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@175344 65c4cc65-6c06-0410-ace0-fbb531ad65f3