Added some fixes to spec
Finalised and tested custom fan outs feature

To use the custom fan outs feature:
1) when creating a new activity pass custom_fanout: true in your
activity params
2) custom_fanout flag is used to avoid triggering
write_activity_in_redis after_create callback for activity
3) call the method fanouts.customised_fanout_for(fans) on the activity
you just created
4) call the method publish on the activity which will do the following:
a) write_activity_in_redis which will write the created activity to the
fans cache.
b) destroy the fanouts objects as they are not needed anymore