Refactoring of the providers and cleanup

The providers do handle the `Identity` not any more. They return only the `LoginInfo` and for the social providers the `SocialProfile` which contains the `LoginInfo` and the profile data returned by the provider. I followed your suggestions for the `Identity` trait. It contains only the `LoginInfo`. The authentication information (secret tokens, password, ...) will only be handled by the providers. I have created an AuthInfoService` which can persist this data into the backing store.

With the current implementation the authentication flow should be as follow.

#### Social Providers:
If the user authenticates the first time, the social provider implementation stores the authentication info for the linked `LoginInfo` into the backing store and returns the `SocialProfile`. Now the controller should create a new `Identity`. Maybe only from the data provided by the social provider or with additional data(IP-Address, UserAgent, Lang, ...). It the user authenticates again at a later time, then the provider updates the authentication info for the linked `LoginInfo` and returns also the `SocialProfile`. Now the identity can be updated with the returned profile data and the `LoginInfo` can be stored in the authenticator cookie.

#### Credentials Provider:
During registration the registration controller stores the authentication info and the created `Identity` in the backing store. Now if the user tries to authenticate with its credentials, the provider checks if the stored authentication info matches with the credentials. If so the provider returns only the `LoginInfo` which can then be stored in the authenticator cookie.