Includable module

Make `InBusiness` includable while supporting the singleton API.

### Example

```ruby
class MyClass
  include InBusiness
end

instance = MyClass.new
instance.hours = {
  monday: "15:00".."18:00"
}
instance.hours.monday
# => "15:00".."18:00"
```

It should respond to all the methods that the singleton API currently have :)