Allow overriding of IMAP server settings on a user by user basis

This patch allows the imapserver, imapport, and imapflags settings to be
overridden for any voicemail user.  It also documents the settings in
the sample voicemail.conf file, and updates the voicemail schema to
allow storage of those columns.

(closes issue ASTERISK-16489)
Reporter: Hubert Mickael
Tested by: Matt Jordan

Review: https://reviewboard.asterisk.org/r/1614/



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@349106 65c4cc65-6c06-0410-ace0-fbb531ad65f3