Added functionality to refresh expired OAuth2.0 access tokens, a oauth session reference class, and using httpuv instead of httpd for OAuth redirect callback

1. added a use_by date-time field to OAuth2.0 access tokens.
2. added a function to check whether a token's use_by date-time has passed (expired) within a specified number of seconds (margin)
3. added a function to refresh an OAuth 2.0 access token
4. added a function to generate an OAuth 2.0 session reference object
5. changed from httpd to httpuv for OAuth 2.0 authentication callback