Implement the package side of the WordprocessingDocument API

Including, but not limited to, 

http://msdn.microsoft.com/en-us/library/documentformat.openxml.packaging.maindocumentpart%28v=office.14%29.aspx
http://msdn.microsoft.com/en-us/library/documentformat.openxml.packaging.wordprocessingdocument%28v=office.14%29.aspx
http://msdn.microsoft.com/en-us/library/system.io.packaging.package.aspx
http://msdn.microsoft.com/en-us/library/system.io.packaging.packagepart.aspx

The aim here is to replace the OPCRelationship, OPCPart and Wordprocessingml classes and the improve the interface used for parsing package relationships.