More flexible invites.

Also supportes now continued invites :)

Fixes #265
Relates candy-chat/candy-plugins#16

commit f5a751cfc78f4db2b5e02fd8b4da070a88e30d56
Author: Michael Weibel <michael.weibel@gmail.com>
Date:   Fri Jul 11 07:56:18 2014 +0200

    Fix naming

commit 72dbba4559531be2696ad1562ee1d5341a4f03cc
Author: Ben Langfeld <ben@langfeld.me>
Date:   Thu Jul 10 11:18:02 2014 -0300

    Support XEP-0249 Direct MUC Invitations

commit c263deede82d5ffc81fc893c6cb8954612b42535
Author: Ben Langfeld <ben@langfeld.me>
Date:   Thu Jul 10 11:04:15 2014 -0300

    Support invites as a continuation of a 1-on-1 thread

commit 85907cb06e8620094c354acabfe18119d5ff4217
Author: Ben Langfeld <ben@langfeld.me>
Date:   Thu Jul 10 10:58:04 2014 -0300

    Support room passwords in MUC invites

commit bc2fa194ce543b5163ff04bd8a66c17fb30a6fea
Author: Ben Langfeld <ben@langfeld.me>
Date:   Thu Jul 10 10:20:48 2014 -0300

    No need to look for this over and over

commit 1532df3482d506df96db786f01dbf5552904247c
Author: Ben Langfeld <ben@langfeld.me>
Date:   Thu Jul 10 10:10:18 2014 -0300

    A mediated invite might be included in a normal message