Feature/717 read api

I'm not sure when `supports` is supposed to do. From the [docs](http://docs.spring.io/autorepo/docs/spring-security/3.1.6.RELEASE/apidocs/org/springframework/security/authentication/AuthenticationProvider.html#supports(java.lang.Class)): 

> Returning true does not guarantee an AuthenticationProvider will be able to authenticate the presented instance of the Authentication class. It simply indicates it can support closer evaluation of it.

It get calls with an `Authetication` class object. That is `ApiUserAuthentication` in our case and we could return false if it's not that exact class (or a subclass).

The TODO comment is an old note for me to remind me what `GoogleAccountsAuthenticationProvider`did in this case and can be removed.