Does Obito master the Summoning Jutsu?
From the the battles of Minato vs Obito and Hashirama Vs Madara I can recollect three important things:-


Both Sharingan users[Obito and Madara] casted genjutsu to Kurama[Kyuubi] and as a result they were able to control it.Kurama was bound to obey them
There was some contract seal between the caster and Kyuubi.
Both Minato and Hashirama used their contract seal.


More about Contract Seal is explained here:-


  Though the Contract Seal removes the summoner's ability to control its summon, it does not give the user of the fūinjutsu the ability to control the summon in turn.


From what I can speculate Obito got some information about the contract seal from Uchiha tablet or directly from Madara.