Fix OAuth token refreshing

[daneden]

This pull request

• Fixes the OAuth2User encoding such that expiration dates are properly encoded (should fix Refresh token issues for oAuth2 #42)
• Improves the demo app by adding a demonstration of how to securely store and retrieve credentials via Keychain (should fix How to go about saving user's credentials?  #40)
• Makes changes to the client credentials trigger Twift.objectWillChange via the @Published property wrapper