Added time_point overloads for creating timestamps. (#128)

* Added time_point overloads for creating timestamps.

* aliased std::chrono types

include/cppkafka/message.h

@@ -240,12 +240,19 @@ public:
     };
 
     /**
-     * Constructs a timestamp object
+     * Constructs a timestamp object using a 'duration'.
      */
     MessageTimestamp(std::chrono::milliseconds timestamp, TimestampType type);
+    
+    /**
+     * Constructs a timestamp object using a 'time_point'.
+     */
+    template <typename Clock, typename Duration = typename Clock::duration>
+    MessageTimestamp(std::chrono::time_point<Clock, Duration> timestamp, TimestampType type);
 
     /**
-     * Gets the timestamp value
+     * Gets the timestamp value. If the timestamp was created with a 'time_point',
+     * the duration represents the number of milliseconds since epoch.
      */
     std::chrono::milliseconds get_timestamp() const;