Initial commit of .NET port of FlatBuffers

Include C# codegen in flatc and .NET FlatBuffer access via the
FlatBufferBuilder class

Tested: on Windows.

Change-Id: If5228a8df60a10e0751b245c6c64530264ea2d8a

net/FlatBuffers/FlatBufferConstants.cs

@@ -0,0 +1,12 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+
+namespace FlatBuffers
+{
+    public static class FlatBufferConstants
+    {
+        public const int FileIdentifierLength = 4;
+    }
+}