Separate setting credentials into a separate class

[samskiter]

It would be useful to have an intermediate class I can subclass that just deals with setting a credential. GROAuth2SessionManager could subclass that.

Then I can use one session manager for all my authenticated calls and another for doing the actual auth.

This is more in keeping with out AFOAuth2Client worked:

it is recommended that you use `AFOAuth2Client` exclusively to get an authorization token, which is then passed to another `AFHTTPClient` subclass.

Unless that advise is now out of date?