return Presence messages in Recv
use `Chat.type` in <message/>'s type attr in Send()

Otherwise client.Send(..) cannot be used for MUC 'groupchat' messages.
This might break code, if you're not setting `Chat`'s type slot and rely
on `client.Send` set it to 'chat' by default
add first simple version of MUC (xep-0045) support