[JDEV] Avatar Update

[temas]

I just got done updating the avatar spec on
http://core.jabber.org/avatar.txt.  Please take a look at it and give us
some feedback.  Notable changes are:

- Defined the public XML namespace to be storage:client:avatar

Reasoning:  I left out the name jabber in any part of this because I
felt that it specified this to be in protocol, and I felt in the future
it might be possible for some storage like this to not be in protocol.
I also chose client over some suggestions for imclient because I feel
avatars could be used elsewhere.  Of course this is still debateable.

- The data tag now has a mimetype attribute

Reasoning:  It helps us more easily support other image formats, and
it's potentially helpful in other ways.

- General updates and cleanups for public xml storage and wording
issues.

--temas