As part of adding this feature:
- fixed bug in update_caps() not assigning verstrings
- fixed xep_0004 typo
- can now use None as a roster key which will map to boundjid.bare
- fixed using JID objects in disco node handlers
- fixed failing test related to get_roster
Several of these bugs I've fixed before, so I either didn't push them
earlier, or I clobbered something when merging. *shrug*
