Be compliant and return 0 from main.

Fixes #713

include/benchmark/benchmark.h

@@ -1301,6 +1301,7 @@ class Fixture : public internal::Benchmark {
     ::benchmark::Initialize(&argc, argv);                               \
     if (::benchmark::ReportUnrecognizedArguments(argc, argv)) return 1; \
     ::benchmark::RunSpecifiedBenchmarks();                              \
+    return 0;                                                           \
   }                                                                     \
   int main(int, char**)