Avatar cache plugin for Tkabber.

This plugin automatically detects avatars announced by PEP messages or
vcard-temp update included in presence. Avatars are downloaded and
stored in an offline cache.

On it's own, the plugin doesn't do anything for the enduser. It is
mostly targeted to other developers, who can write their own plugins
which do actual drawing avatars.

Whenever avatar announce is detected, hook is run:
[hook::run user_avatar_notification_hook $jid $filename]

$jid is a bare form, with stripped resource name
$filename is name of actual file in the avatar cache. It's not warranted 
that it contains any usable graphic data. $filename equal to empty
string means no avatar.

Offline cache is restored on finload_hook. If it's needed at a later
time, it can be invoked again by calling proc avatarcache::restore_cache

