Add APIs to retrieve users and their favorites

Also refactor the pagination metadata and create an object PageRequest to paginate through results.
Modify the Java client to be closer to how the ShopStyle iOS app work so I can use it for unit tests

- When there's no body for a request, the iOS app submits a "null" body instead of an empty JSON object.
- Add support for DELETE requests. Note that DELETE are requests without a body and without a response (at least for now)
Add a hook for the Size API

The API let's you retrieve the sizes that are pertinent for the given category. The category cannot be too high in the category tree or the API will throw an exception.