Refactor

This changeset refactor out the OAuth interface implementations into `OAuthUtility`.  Leaving the component backward compatible (code-wise).

The next commit implements OAuthAuthenticate class that uses the interface in OAuthUtility.  This class can be used to authenticate user by the access_token:

```php
$this->Auth->authorize = array(
   'Form', // standard cakephp login forms
   'OAuth.OAuth', // by access_token in requests (Authorization header or access_token query string)
); 
```

__Note:__ This class only authenticates requests.  Authorization should be handled separately, either by Acl or Controller:isAuthorize()

Since this depends on the previous two PRs, actual diff is: 1913409d7c3c398f117f0d5f4053d7c144f4a983...ac4a4ca13699a1c5b8ddd4c5474df183769e516c