Delegation of methods

This is a backward-incompatible change on top of #61 that removes the need to specify navigational elements when resolving properties. Things like `links` are renamed as `_links` and `embed` renamed as `_embed`, which is what they are called in HAL, but then instead of doing  `api._links.posts_categories` you can do `api.posts_categories` and instead of `api.links.posts.embedded.posts.first`, write `api.posts.first`.

I think this goes a long way into making clients really productive with an API and abstract away the fact that the API is a HAL one. This also opens the door to implementing other JSON format support without needing to rewrite client code.