Create Registrar classes

[jjgrainger]

This PR refactors Post Type and Taxonomy classes by moving the logic to handle registering types to WordPress to their own separate Registrar classes. This is a large change but will help towards making the codebase more maintainable.

• Provide a separation between classes that handle setting data (PostTypes\PostType) and those that handle the logic to register objects to WordPress (PostTypes\Registrar\PostType\Registrar).
• PostType and Taxonomy classes can focus more on interface and providing a clean API for engineers to use.
• "Registrar" classes can focus on the logic of registering post types and taxonomies to WordPress.
• Separation will improve test coverage by separating code that can be tested in isolation and code that requires integration with WordPress.
• Makes the code cleaner to read and easier to maintain.

[jjgrainger]

Close in favour of #97