ami_testhooks.c automatically registers hook

ami_testhooks.c was registering for AMI events upon module load. Moved the registration
to its own CLI command. Added CLI command for unregistering the hook. Changed some of
the wording, removed unnecessary arguments/parameters.

Reported by: rmudgett


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@228661 65c4cc65-6c06-0410-ace0-fbb531ad65f3