plugin events: added eventInvite command as a helper for the complex invitation workflow: when this command is used: - an email invitation is sent with the event node set - invitee's jid is affiliated to event - invitee's jid is affiliated to invitee's node as a publisher - invitee's jid is affiliated to blog's node as a member - invitee's jid is affiliated to every blog's item's comments node as a publisher last affiliation complex operation is currently needed because of the way blogs and comments are handled, but this need to be changed in the future with XEP proposal.