Default behaviour was/is to do all actitivity notifications supported by plugin.
Configure is in the config file by passing array with keyed boolean values like:
addPlugin('Activity', array(
'StartFollowUser' => true,
'StopFollowUser' => false,
'JoinGroup' => true,
'LeaveGroup' => true,
'StartLike' => true,
'StopLike' => false));
Changed the atom activity generation code so it uses the new hooks
built into Notice::asActivity() rather than the hooks in the old
Notice::asAtomEntry().
