migration required before Sept. 1 - 2014

This pull request include some changes needed before September 1 - 2014 as explained here:

https://developers.google.com/+/api/auth-migration#timetable

Please also notice that I've added the default scope to `profile` so we don't break existing examples and documentation.

I've also added some `warns` that might be useful for some folks using this library.

Thanks