src/sugar/activity/activity.py: If the activity is being invoked to
handle an invite, create a Client.Handler instance and share the
activity when HandleChannels is invoked.
src/sugar/activity/activityfactory.py,
src/sugar/activity/activityhandle.py,
src/sugar/activity/main.py: Add a -i switch that indicates to the
activity that it should handle the channel from an invitation.
src/sugar/presence/activity.py: Expose Activity.room_handle.
src/sugar/presence/presenceservice.py: Add get_activity_by_handle().
src/sugar/presence/util.py: Add get_account_for_connection().
