[Core] Multi OAuth providers

Refs #1451.

TODO:
- [x] Rename files: `UserOauth` into `UserOAuth`,
- [x] Fix migration script to not lose existing data,
- [x] Write specs,
- [x] Decide (after feedback) how to cover BC
Hi all, I'm getting this bug where I'm trying to log in via Facebook and where the account has already been registered into the database.

Below is the error 
```php
FatalErrorException: Error: Call to undefined method Sylius\Component\Core\Model\UserOAuth::getRoles() in /Users/warrenlee/Sites/Store/vendor/hwi/oauth-bundle/HWI/Bundle/OAuthBundle/Security/Core/Authentication/Provider/OAuthProvider.php line 84
```

Shouldn't the ``$user`` variable be an instance of UserInterface rather than instance of UserOAuthInterface?