Add new Web flow authentication
Enable web based authentication for all services
Refactor Authenticators to only request id_token if needed.
Avoid error when email is nil.
A0Token & A0UserProfile using Auth0 via Safari.

Fixed fetch profile using id_token. Was using '/userinfo' instead of
'/tokeninfo'.
Handle errors with NSError
Error handling for Auth0 Web authentication.
Small refactor of A0Application & A0APIClient.
Fix issues when cancelling auth flow.
Add social services styling.
Layout changes in login screns.
Some doc changes.
Add missing identity provider icons.
Show error if auth0's scheme is not configured.

To enable Safari Web authentication for non-native providers, the app should
have configured a scheme with the format `a0<ClientId>`.
Merge pull request #23 from auth0/feature-safari-auth

Feature safari authentication