OStatus group delivery initial implementation.

OStatus group delivery initial implementation.
- added rel="ostatus:attention" links for group delivery
- added events for plugins to override group profile/permalink pages
- pulled Notice::saveGroups up to save-time so we can override;
  it's relatively cheap and gives us a clean list of target
  groups for distrib time even with customized delivery.
- fixed notice::getGroups to return group objects as expected
- added some doc on new parameters to Notice::saveNew
 - 'groups' list of group IDs to push to in place of parsing
- messages that come in via PuSH and contain local group targets
  are delivered to local group members
- messages that come in via PuSH and contain remote group targets
  are delivered to local members of the remote group

Todo:
- handle group posts that only come through Salmon
- handle conflicts in case something comes in both through Salmon and PuSH
- better source verification
- need a cleaner interface to look up groups by URI
- need a way to handle remote groups with conflicting names