Added nanoseconds converter

CryptoExchange.Net/Converters/TimestampNanoSecondsConverter.cs

@@ -0,0 +1,29 @@
+using System;
+using Newtonsoft.Json;
+
+namespace CryptoExchange.Net.Converters
+{
+    public class TimestampNanoSecondsConverter : JsonConverter
+    {
+        public override bool CanConvert(Type objectType)
+        {
+            return objectType == typeof(DateTime);
+        }
+
+        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
+        {
+            if (reader.Value == null)
+                return null;
+
+            var ticksPerNanosecond = (TimeSpan.TicksPerMillisecond / 1000m / 1000);
+            var nanoSeconds = long.Parse(reader.Value.ToString());
+            return new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc).AddTicks((long)Math.Round(nanoSeconds * ticksPerNanosecond));
+        }
+
+        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
+        {
+            var ticksPerNanosecond = (TimeSpan.TicksPerMillisecond / 1000m / 1000);
+            writer.WriteValue((long)Math.Round(((DateTime)value - new DateTime(1970, 1, 1)).Ticks / ticksPerNanosecond));
+        }
+    }
+}