Add the option to use a hosted domain with google's oauth

The hd option is used to restrict which email domain that are allowed to
log in to your app.

Starting from this commit you can pass `hostedDomain: 'example.com'` to
only allow emails from the domain `example.com`.
Implement Accounts.config({ restrictDomain: 'mit.edu' })

- Check email for users created with password
- Check email for users created with Google Account
- Check email for users created with Facebook
- Check email for users created with Github
- Set `hd` param for Google Accounts authentication url
- Docs description

- Possibly should add it into QA process?
Don't allow to login w/o valid email by default for all socials.

Keep everything in one place
Doc string from Avi.
Just look for any email we can find.