Fix issue with Delegated Auth response payload

Deprecate [delegationWithRefreshToken:parameters:success:failure:](https://github.com/auth0/Auth0.iOS/blob/1.0.0/Pod/Classes/Core/A0APIClient.h#L166) and [delegationWithIdToken:parameters:success:failure:](https://github.com/auth0/Auth0.iOS/blob/1.0.0/Pod/Classes/Core/A0APIClient.h#L179) because the response will not always match `A0Token`, e.g.: when fetching AWS credentials.

Now there is a generic method to fetch Delegated Auth credentials for another API, it recieves only `A0AuthParameters` and on success returns a NSDictionary with the response payload.
There are two convenience methods to refresh the user's `id_token` either by using a valid `id_token` or a `refresh_token`.