Use newest itsdangerous

In previous version we baked in itsdangerous, but this is not really needed as long as client and server are in sync.

Bake in itsdangerous

Baked in itsdangerous to avoid problems with flask dependency.
See: divio#60 (comment) for context.

Minor dependency updates

Update itsdangerous depedency version.

first release for tagging

1

Singleton pattern #52  patch