Remove OIDCPSessionContext.cs
Merge branch 'dev' of https://github.com/MSOpenTech/azure-activedirectory-identitymodel-extensions-for-dotnet into dev
1. Moved SecurityToken creation from OIDCConfigurationRetriever to JsonWebKeySet.
2. Fixed bug where Base64UrlEncoding should be used instead of Base64Encoding
3. Added tests for Ping.