1. In a small nonreactive saucepan, warm the saffron and wine over moderate heat for 1 minute. In a medium saucepan, boil the shrimp shells with 6 1/2 cups of water over high heat for 5 minutes. Strain the stock into a medium saucepan; keep hot over moderately low heat. In a large nonreactive saucepan, melt 1 1/2 tablespoons of the butter over high heat. Add the shrimp and cook, stirring, until just opaque, about 3 minutes; transfer to a plate. Add the remaining 1/2 tablespoon butter and the onion to the pan and cook over moderately high heat until the onion is translucent, about 2 minutes. Stir in the rice. Add the saffron liquid and boil it until almost completely evaporated.
2. Add 1 cup of the hot shrimp stock or just enough to form a thin veil over the rice. Gently boil the rice, stirring, until the liquid is absorbed. Repeat this procedure, adding the hot stock 1 cup at a time, until it is all used and the rice is tender but firm. Add the shrimp and cook just to warm through. Fold in the tomatoes and season with salt and pepper. Serve at once.