The XXL Survey: I. Scientific motivations - XMM-Newton observing plan - Follow-up observations and simulation programme by Pierre, M, Pacaud, F, Adami, C, Alis, S, Altieri, B, Baran, B, Benoist, C, Birkinshaw, M, Bongiorno, A, Bremer, M, Brusa, M, Butler, A, Ciliegi, P, Chiappetti, L, Clerc, N, Corasaniti, P, Coupon, J, Breuck, C, Democles, J, Desai, S, Delhaize, J, Devriendt, J, Dubois, Y, Eckert, D, Elyiv, A, Ettori, S, Evrard, A, Faccioli, L, Farahi, A, Ferrari, C, Finet, F, Fotopoulou, S, Fourmanoit, N, Gandhi, P, Gastaldello, F, Gastaud, R, Georgantopoulos, I, Giles, P, Guennou, L, Guglielmo, V, Horellou, C, Husband, K, Huynh, M, Iovino, A, Kilbinger, M, Koulouridis, E, Lavoie, S, Brun, A, Fevre, J, Lidman, C, Lieu, M, Lin, C, Mantz, A, Maughan, B, Maurogordato, S, McCarthy, I, McGee, S, Melin, J, Melnyk, O, Menanteau, F, Novak, M, Paltani, S, Plionis, M, Poggianti, B, Pomarede, D, Pompei, E, Ponman, T, Ramos-Ceja, M, Ranalli, P, Rapetti, D, Raychaudury, S, Reiprich, T, Rottgering, H, Rozo, E, Rykoff, E, Sadibekova, T, Santos, J, Sauvageot, J, Schimd, C, Sereno, M, Smith, G, Smolčić, V, Snowden, S, Spergel, D, Stanford, S, Surdej, J, Valageas, P, Valotti, A, Valtchanov, I, Vignali, C, Willis, J, Ziparo, F