Merge pull request #11 from ThomasBarnekow/feat/visual-studio-project

Visual Studio Project for Open XML SDK (without any of the following commits)
Implemented Save and Clone methods
Now implementing factory methods, e.g., CreateClone, in sublcasses of OpenXmlPackage contained in PackageDocument.cs
Checked null references and added documentation.
Added SaveAs(string) method. Checked FileOpenAccess == FileAccess.ReadWrite in Save() method to avoid exception in Package.Flush().
Added Flat OPC conversion methods to OpenXmlPackage, WordprocessingDocument, SpreadsheetDocument, and PresentationDocument.
Currently, all classes convert to and from XDocument but only WordprocessingDocument also converts to and from Flat OPC string.
Some testing has been performed.
Completed and tested Flat OPC conversions