CONTRIBUTION AGREEMENT BY AND AMONG EL PASO CORPORATION EL PASO LLC EL PASO NORIC INVESTMENTS III, L.L.C. COLORADO INTERSTATE GAS COMPANY, L.L.C. EL PASO CNG COMPANY, L.L.C. EL PASO CHEYENNE HOLDINGS, L.L.C. CHEYENNE PLAINS INVESTMENT COMPANY, L.L.C....