Added GPU instancing for DirectX

Works on HiDef profile on DirectX projects. Has a difference with XNA framework interface: instead of doing this:
_device.SetVertexBuffers(
 new VertexBufferBinding(vertexBuffer, 0, 0),
 new VertexBufferBinding(instanceBuffer, 0, 1)
);
you should do it like this:
_device.SetVertexBuffer(vertexBuffer);
_device.SetInstanceBuffer(instanceBuffer);

Other API parts of instancing is same as in XNA.