[WIP] Port to netstandard2.0

[616b2f]

The new PR as we discussed in #12

things TODO (to provide same functionality as before):

• Include new commits from main project
• can ADFS import the metadata?
• can ADFS use this as a claims provider
• can SharePoint use this as an IdP
• implement encryption for assertions (waiting for Saml2EncryptedAssertion AzureAD/azure-activedirectory-identitymodel-extensions-for-dotnet#1025)

TODO (additional features):

• provide better unittests
• add 'offered claim types' to metadata file
• add wfresh="0" support (see http://docs.oasis-open.org/wsfed/federation/v1.2/os/ws-federation-1.2-spec-os.html)