Split core infrastructure into separate directory

[rmccue]

From #571. This should make it easier to review the infrastructure, and is our first step towards actually getting this stuff reviewed.

[rmccue]

Basically, we want to take the API and split it into two sections: the infrastructure, and the endpoints. The infrastructure basically encompasses the server/router, request/response objects, and the main plugin.php file. We want to take those and move them into something like lib/infrastructure and the rest into lib/endpoints.

Essentially, it's everything you need to create an API, but not the user facing API itself.

[danielbachhuber]

@rmccue I've broken out /lib/infrastructure and /lib/endpoints. Not clear what we want to do with plugin.php or lib/wp-json.php right now — can you specify?