Added GetProfilePicture

Added GetProfilePicture: fetches profile picture and stores it in
pictures/[JID].jpg
for some reason only thumbnails work at the moment, need to look into
this.

Fixed SendPresence() sending empty name attribute when no nickname was
provided, resulted in xml error protocolnode.
Included GetProfilePicture() output folder

Added placeholder in /src/php/pictures in order to add output folder to
repository
Updated testprotocol.php

Added GetProfilePicture() to retrieve large profile picture, fore some
reason it magically started working again :o
Replaced five Message() calls with just one to protect user from being
blocked.
Reverted changes to Nickname

Nickname mandatory again, resulted in xmlformatting errors.