Adds new API endpoints for elements and pages

Adds four new API endpoints for loading Alchemy::Elements and Alchemy::Pages via json:

* /api/pages.json
=> Returns all public pages. Pass page_layout to only return pages with given page layout

* /api/elements.json
=> Returns all public elements

* /api/pages/:page_id/elements.json
=> Returns all public elements for given page_id

* /api/pages/:page_id/elements/:named.json
=> Returns all public elements for given page_id and with given name.