Use `Auth::get_user_pass` in `Client::new_auth`

[ValuedMammal]

It would make sense to use Auth::get_user_pass in the implementation of Client::new_auth, rather than re-implement reading a file.

[ValuedMammal]

Out of curiosity, what is the purpose of the Auth::None variant?

[tvpeter]

Out of curiosity, what is the purpose of the Auth::None variant?

I initially thought that it was possible for a user to configure their node not to use any auth method, but having checked again, it is not needed.