Update all astakos API methods to use the new common 'api_method'
decorator. The 'api_method' decorator 'user_required' option
authenticates uses by their token through astakos. Consequently,
astakos methods can not use this option. Instead, two custom
decorator are implemented for astakos:
* user_from_token: Asserts that a user with the specified X-Auth-Token
  exists
* service_from_token: Asserts that a service with the specified
* X-Auth-Token exists.