A few fixes to groupchat draft

- Minor editorial fixes
- Fix the "private-messages" value on a=chatroom attribute
- Removed usage of nickname-text (I'll elaborate on this on an upcoming pull request).
- Specify that the server should cache the participants list until the SIP subscription has been completed.

Next I plan to add is a more complete mapping between RFC4575 and presence stanzas and how the display-text and nickname (from 4575) elements should be used.