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

tests/MyGame/Example/Test.cs

@@ -0,0 +1,24 @@
+// automatically generated, do not modify
+
+namespace MyGame.Example
+{
+
+using FlatBuffers;
+
+public class Test : Struct {
+  public Test __init(int _i, ByteBuffer _bb) { bb_pos = _i; bb = _bb; return this; }
+
+  public short A() { return bb.GetShort(bb_pos + 0); }
+  public byte B() { return bb.Get(bb_pos + 2); }
+
+  public static int CreateTest(FlatBufferBuilder builder, short A, byte B) {
+    builder.Prep(2, 4);
+    builder.Pad(1);
+    builder.PutByte(B);
+    builder.PutShort(A);
+    return builder.Offset;
+  }
+};
+
+
+}