Message needs a field to expose senders full JID

[Martin Morrison]

When handling a Message, the sender_full field refers to:

 - The full user JID when handling a chat
 - The bare JID when handling a muc

It appears that the sender field always refers to the bare user JID.

There needs to be a way to access the full user JID in a room (so that plugins can handle things on a Resource-by-Resource basis should they wish to).

This bug makes the trains plugin useless for tracking "nearest station" for users who wish to query it from within a room.