add 'favorite_other' event type

Hello. I'm thank you for this fantastic gem.

When I make a twitter datastore application, I want to handle 'user favorites' event via Chatroid API. So, I made this pull request.

By this change, user can call `on_favorite_other` method in Chatroid instance.
```ruby
Chatroid.new do
  on_favorite_other do |event|
    "some codes"
  end
end.run!
```

I'm worry about this event type name, 'favorite_other'. This is not cool, but `on_favorite` is booked as 'user is favorited' event. If you have good idea for this, please feel free to change.


And I'm beginner on making pull request, maybe I made mistakes. If you found any fixes on my pull request, please tell me.

Thank you.