Request bookmarks stored in PubSub

According to XEP-0048 v1.1

Fixes #266

commit b0da970c21bc8505443c5f3ab765c19d47c8d654
Author: Ben Langfeld <ben@langfeld.me>
Date:   Fri Jul 11 12:51:52 2014 -0300

    Request bookmarks stored in PubSub according to XEP-0048 v1.1

    Fixes #266