Removed display-text / nickname open issue

Clarify that nickname takes precedence in case both are present
Mention that goodbye messages should be broadcasted as well
Added missing a:chatroom SDP attribute in answer
Use empty a:chatroom attribute to indicate the target is a groupchat

draft-simple-chat does define it also for offers, even if not as a
negotiation mechanism:
http://tools.ietf.org/html/draft-ietf-simple-chat-18#section-8

Either way, it expresses local policy, so when a MSRP client sets it in
the offer it means it's willing to use capabilities defined by
draft-simple-chat.
Fixed room URI in NOTIFY example
Added open issues about use of participant JIDs in semi-anonymous rooms
Change SHOULD for MUST

There is no other way for changing the nickname as per
draft-simple-chat, so a NICKNAME request must be used
Added initial explanation on how to invite participants to a room
Fixed misplaced section after merges
Fixed MSRP response code for nickname conflicts