Changed to .NET Standart

• added .net standart 2.0 as target framework
• added ef core v2.0.0 as default framework version
• added unit tests for v2.2.6 and v3.1.1

multitarget support

Added support for:

• net461 -> Microsoft.EntityFrameworkCore v2.x
• netcoreapp2.0 -> Microsoft.EntityFrameworkCore v2.x
• netcoreapp3.0 -> Microsoft.EntityFrameworkCore v3.x