Logger: Add dynamic logger channels

This adds the ability to dynamically add and remove logger channels
from Asterisk via the CLI.

(closes issue AST-1150)
Review: https://reviewboard.asterisk.org/r/3185/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@407989 65c4cc65-6c06-0410-ace0-fbb531ad65f3