This checks if the username/password combination for server is available in the secure storage.
Resolves to `true` if an entry exists or `false` if it doesn't.
* First draft of implementing secured storage support (TouchId or Passcode)
* minor improvements
* improving the implementation; Support for AppDelegate-notification
* minor changes and improvements
* provding requested constant as usability feature; added documentation to canImplyAuthentication
* updating .d.ts-file
* when fetching stored items using the traditional modality (not TouchId or Passcode protected) ignore any items that need authentication.
