Send multiple MUC invites in single stanza for one2one_chat_to_muc

This corresponds more closely to example 63 in XEP-0045 (version 1.25),
and is relevant to esl/MongooseIM#176.
Merge pull request #49 from legoscia/muc-multi-invite

Send multiple MUC invites in single stanza for one2one_chat_to_muc