Support for Twitter's reverse authentication

Adds support for https://dev.twitter.com/docs/ios/using-reverse-auth
This change tweaks the OAuth.getOAuthRequestToken as follows:
1. if  {x_auth_mode: 'reverse_auth'} parameter is passed in as extraParams, then the callback parameter is not included with the request.
2. the callback passes a 5th parameter which is the complete unparsed raw data received, since that whole string is required as-is for step two on the device.

I've also updated the readme file with an example and added a test. This does not break regular authentication.