- added APIAccess class to define access levels of API Endpoints (permissions)
- use "mailpoet_token" for all nonce (just as before)
- merged setupPublic/setupAdmin methods in API in order to avoid duplication
- check permission if access level is not all
- fixed ABSPATH check in some classes
- (re)Added Endpoints folder to both API and Router
- fixed syntax in namespaces
- xhr.responseJSON is returned to the fail()
- fixed Router endpoints (view in browser, cron,...)