preparing v3.1.1 release

release package to nuget

update tools for release

preparing release

tools update

Merge pull request dvsekhvalnov#120 from dvsekhvalnov/rfc_7797

Rfc 7797: unencoded and detached content support

bump up version to 2.4, prepare .nuget

bump up version to 2.3

dvsekhvalnov#35 bump up dependencies version to latest, cleaned up de…

…pendency on NETStandard.Library

bump up version to 2.1