Better refreshing of OAuth tokens when using class methods

If you're using class methods such as `CreateSend::Subscriber.add`, doing something like this:

```ruby
auth = {
  :access_token => 'access token',
  :refresh_token => 'refresh token'
}
begin
  tries ||= 2
  CreateSend::Subscriber.add auth, list_id, email, name, custom_fields, true
  rescue CreateSend::ExpiredOAuthToken => eot
    access_token, expires_in, refresh_token =
      CreateSend::CreateSend.new(auth).refresh_token
    retry unless (tries -= 1).zero?
    p "Error: #{eot}"
end
```

It's annoying that you can't refresh an OAuth token using a class method (in this case you have to do `CreateSend::CreateSend.new(auth).refresh_token`). Add a class method `CreateSend::CreateSend.refresh_access_token(refresh_token)` to make this easier.