Zanzibar Revolution: Who was John Okello and what did he announce on the radio in 1964?

Zanzibar and Tanzania mark 50 years since the bloody revolution of Zanzibar in 1964. I am currently researching on the Tanganyika- Zanzibar Union and the Zanzibar Revolution of 1964 has certainly shaped the history of Tanzania. I recently came across someone's tweet saying "Bila Mapinduzi, Muungano usingekuwepo..bila ya Muungano, Mapinduzi yasingedumu." The English translation is "Without the Revolution, the Union could not exist..without the Union, the Revolution could not have held." This is indicative enough of the crucial nature of the Zanzibar Revolution to the history of Tanzania. 

Strange as it may seem, one author aptly notes that "the Zanzibar Revolution might not have occurred, and certainly not when it did, had it not been for the presence of a foreigner from Uganda who had grandiose illusions about his revolutionary calling and who happened to be in an ideal position to organise a coup. The man was John Okello, a Christian who had come to Pemba as a youth  to make building blocks. He joined the Police Force and in 1961 became one of the leaders of the ASP party. In February 1963, he went over to Zanzibar island and began to work work for the realisation of his dream of becoming a great nationalist by overthrowing the Sultani. Okello was an eloquent young man with a deep harrowing voice. After periods of disillusionment he decided to join with the ASP Youth League. He styled himself as Field Marshal. In the early morning of 12 January 1964 sabotaged the police arsenal and together with his men defeated the Sultan's troops. At 7 a.m Okello went to the radio station and broadcast the following message to the people of Zanzibar:

I am Field Marshal Okello! Wake up, you imperialists, there's no longer an imperialist government on this Island; this is now the government of the Freedom Fighters. Wake up, you black men. Let everyone of you take a gun and ammunition and start to fight against any remnants of imperialism on this island.

Later that morning, Okello issued an ultimatum to the Sultan:You are allowed twenty minutes to kill your children and wives and then kill yourself. The Sultan escaped to Tanganyika then to England. Okello began to form a provisional government with Abeid Karume as Prime Minister. By the evening the Revolution was full circle and running.