.NET Core Compatibility

[qmfrederik]

Hello!

This pull request adds .NET Core Compatibility for PNGDecrush, allowing you to use it on a new version of the .NET Platform. (As a sidenote, this also brings .NET to Linux, so it may also help address #4).

The changes mainly boil down to:

• Removing the dependency on DotNetZip, whic is not yet available on .NET Core
• Manually generating the zlib stream around the deflate stream (a zlib stream is a deflate stream with a header and a footer)
• Copying over the CRC32 algorithm from DotNetZip
• Minor compatibility fixes, such as removing the using System.Web statements which would case compilation issues.

I've also added an AppVeyour build script which provides CI capabilities for this repository.

Hope it helps!