OStatus partial support for group subscriptions:

OStatus partial support for group subscriptions:
* detection of group feeds is currently a nasty hack based on presence of '/groups/' in URL -- should use some property on the feed?
* listing for the remote group is kinda cruddy; needs to be named more cleanly
* still need to establish per-author profiles (easier once we have the updated Atom code in)
* group delivery probably not right yet
* saving of group messages still triggering some weird behavior

Added support for since_id and max_id on group timeline feeds as a free extra. Enjoy!