Save and Clone

This pull request only contains the four commits containing the Save, SaveAs, and Clone methods added to OpenXmlPackage, WordprocessingDocument, SpreadsheetDocument, and PresentationDocument. 

It does not contain the Flat OPC conversion functionality which we might still want to discuss with regards to the FlatOpcPackage class that I implemented in my Open-XML-Extensions repository (see https://github.com/ThomasBarnekow/Open-XML-Extensions).