Module requires curl but it is not declared in hook_requirements

I'm not really sure it should require curl at all, since drupal_http_request is available in core (and thus on any Drupal installation). But since I don't know the limitations of drupal_http_request and don't have the time to check what DrupalOAuthClient is doing with curl, I'm providing a patch to warn new users that curl is a requirement for oauth_common.