Change when google-signin-aware requests auth

By requesting in domReady, we know all our children (the signal handlers) are ready before we call into google-signin code.
Merge pull request #48 from hjfreyer/patch-1

Change when google-signin-aware requests auth
Removing shared global variables for some of the general attributes.

This makes it so that the attributes can be changed like normal by the DOM and have it affect the login. Ex: adding more scopes after adding to DOM.

Also a step toward not having multiple signin buttons share common state on element specific information.
Removing more dependencies on global variables.

Instead of relying on global variables for handling the signin moving to have each button be able to manage their own state.
Updating the handling of the `authResult` to be done by all buttons.

Triggering for auth related events are handled by a default handler (first button attached).

Each button handles the auth result and updates its properties depending on the scopes granted or remaining to be granted.
Updating the demo page with more examples of using the signin button.
Updating the scopes in the example to show another use case.

Shows the effect of using multiple scopes that other buttons also grant portions.
Fixing a bad attribute for the email scope.
Fixing the native style events to still fire, just the signals need to be restricted.
Fixing the failed auth from cancelling consent.

When a user cancels a consent dialog it triggers all the buttons to update, but the buttons were all defaulting to signed out state since there was any error given in the result.
Switching to the profile.emails.read scope and the email scope.