Prepare version 2.6.0 - #37 #38

Specify timeout in msec instead of sec

Introduce warning level

Conditional tests execution

Use windows instead of ubuntu to meet .NET 4.6.1 requirements

Revert "Test .NET Standard with no dependencies"

Use Portable.BouncyCastle version for .NET Standard

Prepare next rel. version

Migrate library .NET Core -> .NET Standard