Add new .NETPortable class-library build of SgmlReaderDll.

This commit creates a new c# project that targets the .NETPortable runtime including .NET 4.5, Silverlight 5, Windows Phone 8 and .NET for Windows Store apps. This allows the client to be used by projects targeting those frameworks.

Define Constants are used to selectively enable functionality not present in these lighter frameworks.

I didn't create a new nuspec for this library because it would require additional changes to the PSake build scripts to accomplish and felt someone else might know how to do this more quickly than I can.