Added oauth_callback param to the request, which is required for OOB (out-of-band) authentication

Without oauth_callback param twitter does not return a pin but
redirects to the callback url set in the app on twitter.com, hence
failing the authentication. More at:
https://dev.twitter.com/docs/auth/pin-based-authorization
Merge pull request #115 from YasKuraishi/master

Added oauth_callback=oob to the request as request gets redirected to callback url set in the twitter app instead of returning the pin.