Add uid/info/extra to the strategy

@ricardochimal pointed out we're not complying with the [omniauth auth hash spec](https://github.com/intridea/omniauth/wiki/Auth-Hash-Schema), as `info` is required (and it must have a `name`).

We originally took these out because of the additional API call. I'll bring it back optionally so users don't have to do [things like this](https://github.com/heroku/ion/blob/master/app/controllers/auth_controller.rb#L52) when all they want is the user email.

Some pulls are already addressing this, but I'm starting from scratch to use Faraday (which is bundled with omniauth) instead of heroku-api (which brings Excon). This way we have less dependencies, and won't cause issues if users are already using these gems on different versions.