made optional queryable modules be loaded to gem namespace by default

Hi there.
I'm not sure if that is desired behaviour, but each time one wants to use `Queryable::DefaultQuery` module, he should manually `require` that module in his query file, like that:
```ruby
require 'queryable/default_query'
class LocationsQuery
  include Queryable
  include Queryable::DefaultQuery # this line won't work without require
end
```
In my case I had to spend some time to manage this out (as it's not mentioned in README), and it's probably gonna end with tons of that `require` statements throughout my query files (which is kinda not DRY).
I suggest to require that modules inside of main `queryable.rb` (or at least make some addition to README that it's what it intended to be).