Auto merge verified accounts

Creation of the new user accounts when the `auth_id` wasn't found is changing to:

- If verified provider (Google, Yahoo!, etc) check first if that email already exists and it's verified.
- If it exists then add that `auth_id` to the existing user and return that one
- if it doesn't exist.. do the usual :)

It will work with a combination of #200 and won't be merged as a stand alone PR.