ProtoXEP: Update explicit mentions

[snit-guckfung]

I've received mixed feedback on when I should submit changes, but I've changed a lot already, so I figure I'll submit what I have now.

Changes include:

• mentions -> type and is now required (since we aren't using refs)
• jid/occupantid/uri -> target for consistency
• begin/end are now mandatory and hopefully more clear what they do
• Deleted hreflang in favour of xml:lang
• Deleted <noping /> since no one wanted it
• Made active a boolean attribute since I don't see a use for any other modifiers
• Deleted space and server mentions; they were weird and can be specified later if needed
• Room mentions no longer get a target; always current room
• Role/Affiliation mentions now use one type with target containing the role/affiliation
• Permissions now use muc#roomconfig and servers MUST act based on it
• Encourage including characters like '@' in begin/end if sent

As with the initial submission, I have an example Slixmpp plugin in Python located here.