Change scopes after initialization

[igor-shevchenko]

In my application, there are some optional scopes that are not requested when a user signs up. I ask for them later on when the user decides to use the specific feature. To implement this with angular-google-plus I need to be able to change requested scopes after GooglePlus is initialized.

I've added two new methods to GooglePlus that allow getting and setting scope value for new login requests.