South Africa hosts the third annual Tech4Africa conference, in Johannesburg, attracting innovators and entrepreneurs from a dozen countries. Among the speakers are Sim Shagaya, a Nigerian-born Harvard graduate planning to create the 'Amazon of Africa', selling Lagos's increasingly affluent consumer class everything from refrigerators to perfume to cupcakes. His previous venture, DealDey, which offers Groupon-style deals, is now the top-grossing ecommerce site in Nigeria with 350,000 subscribers.