Issue title: Add ability to do reverse auth for oauth
 Issue body: Twitter for iOS needs reverse auth if you'd like to store the credentials on the server

https://dev.twitter.com/docs/ios/using-reverse-auth

It simply needs the ability to add  x_auth_mode: 'reverse_auth' to the request token.

 Comments: 
 Comment 0: Can you please elaborate on your requirements?

 Comment 1: Hi!

Sorry my explanation was poorly done. The OAuth module implemented now is very strictly for the web oauth and very restrictive.
I simply needed the ability to add a header for the request token portion in order to implement reverse-auth (see link in original message).

I ended up with something like the following:

```
/** *
* The following is the reverse oauth step outlined by twitter
* This is needed for iOS applications in order to send back
 * the oauth token for the user without storing our app secret
* on the device
* reference: https://dev.twitter.com/docs/ios/using-reverse-auth
*/
def reverse = Action.async {

val client = new DefaultHttpClient();
val params = new java.util.ArrayList[BasicNameValuePair](1)
params.add(new BasicNameValuePair("x_auth_mode", "reverse_auth"))
val consumer = new CommonsHttpOAuthConsumer(KEY.key, KEY.secret)
val post = new HttpPost(TWITTER_SERVICE_INFO.requestTokenURL)
post.addHeader("Content-Type", "application/x-www-form-urlencoded")
post.setEntity(new UrlEncodedFormEntity(params))
consumer.sign(post)

val futureBodyString: Future[String] = future {
val response = client.execute(post)
  EntityUtils.toString(response.getEntity())
}

futureBodyString.map {
  body => {
    val jsonResponse = JsObject( Seq( ("x_reverse_auth_parameters", JsString(body)) ) )
    Ok( jsonResponse )
  }
}
```

  }

 Comment 2: This looks like an application feature.

{'source': 'GithubIssues', 'repo_name_with_owner': 'playframework/playframework', 'issue_id': 26148484, 'repo_stars': 12248.0}