Replace use of SharpZipLib with standard .NET DeflateStream

SharpZipLib is used in only one place, to inflate zTXt chunks in PNG files.
DeflateStream can do the same job, without introducing a dependency
or complicating nuget packaging.
Since DeflateStream is a built-in .NET class, there is no longer a reason to
make zTXt support conditional, so I removed the conditional compilation.
Also removed a small defect in the VS project file which prevented compilation using VS.
Decode hex-encoded tags in PNG files

especially XMP data (ImageMacgick)
Fix assembly signing

Making a release build in Visual Studio requires the correct
path to the snk file in AssemblyInfo.cs
Bump version number to reflect new feature

and to distinguish from official release
Merge pull request #1 from  JohnThomson/hexXmp