If you read the manga that was released today you discovered for certain that the man who claimed to be "Tobi" isn't in fact Madara. With that being said I believe that the man behind the mask is Izuna Uchiha, Madara's little brother. I say this, because in today's manga chapter Madara made a direct reference to "Tobi" in such a form as if he knew him personally. For example Madara was thinking to himself that reviving him was something he would do. I would say he made about three references in this manor. Izuna Uchiha would also be a good guess, because he had the mangekyou sharingan before he contributed his eyes to Madara, which would explain how he's able to control a sharingan. (And if I may add, I think Izuna may have gotten that new sharingan from finding Obito's body. Obito gave his left eye to Kakashi ... if you haven't noticed ... "Tobi" or Izuna has his sharingan in his right eye. Sure Obito's right side got crushed, but that doesn't entirely mean that his right eye was destroyed does it? I don't think so) Oh and to explain him being able to maintain both a sharingan and a rinnegan? Well he's an Uchiha and plus he found some way to stay alive for so long, so why couldn't he control a rinnegan? Also we learned today that a sharingan user could eventually unlock and begin to utilize the rinnegan as Madara has done.