Prototype

Here is the first steps of the prototype, it's a rough draft of my idea.

Commands that you should try and should work from the console:

```bash
# enter the console
cd spec/dummy && rails c

p = Post.create(user: User.first)
a = ChooChoo::Activity.create(master_event: p)

# get all the activities with the latest one first
ChooChoo::Activity.order(updated_at: :desc)

# get the top node of the last activity
ChooChoo::Activity.last.master_event

# get the model the latest event happened to
# will currently always be the same as master_event
ChooChoo::Activity.last.last_event
```