Handle multiple MUC invites in single stanza

This is what's found in example 63 in XEP-0045 (version 1.25),
and also what the schema specifies (unbounded number of 'invite'
elements).

This closes esl/MongooseIM#176.
Merge pull request #180 from legoscia/muc-multi-invite

Handle multiple MUC invites in single stanza