Allow Skin.AddAttachment to overwrite safely

Allow Skin.AddAttachment to overwrite an existing attachment as intended.
( For more info, see the Remarks section of: http://msdn.microsoft.com/en-us/library/k7z0zy8k%28v=vs.110%29.aspx )
Merge pull request #198 from pharan/patch-1

Allow Skin.AddAttachment (C#) to overwrite safely