Make library available as a nuget package that supports .NET Standard and .NET Framework

[lassevk]

Goal:

• Support .NET Standard 1.x
• Support .NET Standard 2.x
• Support .NET Framework 4.x

where "x" above in each instance is the lowest value that makes sense, to support the broadest available platforms.

[rodrigocaballerohurtado]

Hey @lassevk, any plans to turn this lib into dotnet core compatible? thanks