Added a .NET Standard project

[TecVector]

This will enable this to be built into .NET Core and newer .NET Framework versions.

[elerch]

How different is the SAML.Standard different from SAML2.Core? It looks like much of the new project is copied over, but it would help me if you can point at the significant differences.

[TecVector]

It is mostly copied over - the main differences were just removal of some of the references and api calls that still had a reliance on the .NET framework. I ran the .NET profiler to see what the compatibility was, and followed those results. After successful completion, I was able to load that DLL into a .NET Core and Standard project w/o having any conflicts about how it was compiled. This was an issue I was seeing with the original SAML.Standard project. Most of the functionality is still in original form, so it is minor tweaks.