Added picture subscription

ALL HAIL MISSVENOM!
This is some weird shit right there. It would appear that WhatsApp does
in fact have some kind of server-side contact list per user. Using this
subscription will send you a notification when someone's picture has
been changed, even if you never had a conversation with them.
I tested this on my dev number, imported and synced my Google account
with WhatsApp, but never had a conversation with that contact on this
number.
This is what the notification looks like:
rx<message from="316********@s.whatsapp.net" id="575363392"
type="notification" t="1369645399" retry="1">
rx  <notify xmlns="urn:xmpp:whatsapp" name="***** *******"></notify>
rx  <request xmlns="urn:xmpp:receipts"></request>
rx  <notification type="picture">
rx    <set jid="316********@s.whatsapp.net" id="1369645399"></set>
rx  </notification>
rx  <offline></offline>
rx</message>

Still need to add an event for this :)