How do you go head-to-head with the world's soft-drink kings -- Coca-Cola (KO) and PepsiCo (PEP) -- and challenge the slew of smaller, innovative makers? Maybe by creating a system that allows consumers to make their own sodas. SodaStream (SODA) has done exactly that. The Israeli company's nicely priced gizmo allows the thirsty to turn tap water into sparkling water and, from there, into soda (regular or diet), lemonade, energy drinks or sparkling