Protocol GS-US-248-0131: A Phase 2 Randomized, Double-Blind, Placebo-Controlled Study of GS-5885, GS-9451, Tegobuvir and Ribavirin (RBV) Compared with GS-5885, GS-9451 with Tegobuvir or RBV in Treatment-Experienced Subjects with Chronic Genotype 1a or 1b