Adding feature to support the storage and retrieval of voicemail greetings using IMAP storage.

This feature may be turned on by adding imapgreetings=yes to the general section of voicemail.conf
voicemail.conf.sample has details on the options added.

As a result, IMAP storage now has RETRIEVE and DISPOSE macros defined.

In addition to the IMAP greeting changes, I also have added an enum for the voicemail folders
and so now the code should be easier to understand and maintain when it comes to this area.



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@72232 65c4cc65-6c06-0410-ace0-fbb531ad65f3