add macOS implementation

macos/include/TimeIt.hpp

@@ -0,0 +1,32 @@
+#ifndef TIMEIT_H_
+#define TIMEIT_H_
+
+#pragma once
+
+#if __cplusplus <= 199711L
+ #include <ctime>
+#else
+ #include <chrono>
+#endif
+
+class Timer
+{
+ public:
+
+  Timer() { reset(); }
+  void reset();
+  double elapsed();
+
+ private:
+
+#if __cplusplus <= 199711L
+  timespec beg_, end_;
+#else
+  typedef std::chrono::high_resolution_clock clock_;
+  typedef std::chrono::duration<double, std::ratio<1> > second_;
+  std::chrono::time_point<clock_> beg_;
+#endif
+
+};
+
+#endif  // TIMEIT_H_